Observations of a perigee coincident with a full Moon

Observations of a perigee coincident with a full Moon

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Hopefully this will be a quick and easy question, with a quick and easy answer, but why is it that we observe a perigee Moon coincident with a full Moon approximately every 13 months? Is this related to the Earth-Moon dynamics, perhaps the eccentricity of the lunar orbit?

The time between full moons is called the the synodic month, and the time between perigees is called the anomalistic month. The anomalistic month is shorter than the synodic month by about two days, (27.6 days and 29.5 days)

Since the anomalistic month is two days shorter, and there are nearly 28 days in a month, it takes about 14 months for the two cycles to come back into alignment. Thus there is a perigee at full moon every 14 (not 13) months.

More accurately the beat period between the two cycles is 411.78443 days. See wikipedia:

The 10 must-see night sky events to see in 2021

Another year of skywatching is upon us, and there's a lot to look forward to in 2021!

Here are the 10 most noteworthy sky events. A close pairing-off between two bright planets, a total and nearly total eclipse of the moon, a great year for viewing the beloved Perseid meteors and a great autumn appearance for Venus are among the celestial highlights that will take place in the new year.

Of course,'s Night Sky column will provide more extensive coverage of these events as they draw closer.

What is a perigean spring tide?

Often between 6-8 times a year, the new or full moon coincides closely in time with the perigee of the moon — the point when the moon is closest to the Earth. These occurrences are often called 'perigean spring tides.' High tides during perigean spring tides can be significantly higher than during other times of the year.

In order to understand the phenomenon called a 'perigean spring tide,' you first have to know that the gravitational pull of the moon and the sun cause tides. Tides are actually long-period waves that roll around the planet as the ocean is 'pulled' back and forth as the moon and the sun interact with the Earth in their monthly and yearly orbits.

The next thing you need to know is that the moon follows an elliptical path around the Earth in its monthly orbit, and the Earth follows an elliptical path in its yearly orbit around the sun. This means that, at times, the moon and the sun are closer to Earth. At other times, they are farther away. What happens when the moon and the sun are close to the Earth? You guessed it: the gravitational pull they exert is stronger, resulting in slightly higher tides.

While both the moon and the sun influence tides, the moon plays a much larger role because it is so close to the Earth. Its gravitational pull is about twice as strong as that of the sun. Now consider these two cases:

The Effect of a Full or New Moon

During full or new moons — which occur when the Earth, sun, and moon are nearly in alignment — average tidal ranges are slightly larger. This occurs twice each lunar month (about 29.5 days on average). The moon appears new (dark) when it is between the Earth and the sun. The moon appears full when the Earth is between the moon and the sun. In both cases, the gravitational pull of the sun is &lsquoadded&rsquo to the gravitational pull of the moon on Earth, causing the oceans to bulge a bit more than usual. This means that high tides are higher and low tides are lower than average. These are called 'spring tides.'

NOAA's High Tide Bulletin and Flooding Reports

There are many factors that cause the tides to be higher than what is "normally" seen from day to day. Our seasonal bulletin tells you when you may experience higher than normal high tides where you live. We also publish annual high tide flooding reports that present a broad outlook of what to expect for a given year in terms of high tide flooding, as well as a summary of high tide flooding events for the previous calendar year.

The Effect of Perigee

Once about every 28 days, the moon reaches a 'perigee,' its closest point of approach to the Earth. This is the point at which the gravitational pull of the moon is strongest. During these periods there will be an increase in the average range of tides. Conversely, about 14 days following the perigee, the moon reaches an ‘apogee’, its furthest point of approach to the Earth. This is the point at which the gravitation pull of the moon is weakest. During these periods there will be a decrease in the average range of tides.

What happens when a full or new moon coincides with perigee?

Full or New Moon + Perigee

Typically between 6-8 times each year, the new or full moon coincides closely in time with the perigee of the moon — the point when the moon is closest to the Earth. These occurrences are often called 'perigean spring tides.' The difference between perigean spring tides and spring tides that occur closer to the moon’s apogee are location dependent and significantly influenced by tidal range, but can be quite large. It is not uncommon for high tides during a perigean spring tide to be more than a foot higher than high tides during ‘apogean spring tides’. In places like Anchorage, Alaska, which has a tidal range over 30 feet, the difference between spring tides can be 3 feet or more at high tide!

It's also important to note that other factors influence the height of the tide as well. Seasonal effects on mean water level and the tide, like higher water level due to the thermal expansion of warmer water, can sometimes mean that some of the highest tides of the year are not perigean spring tides.

six inches . But considering that these areas have an average tidal range of more than 30 feet , the increase is but a small percentage of the whole (less than a two percent increase).

Perigean Spring Tides and Coastal Flooding

Coastal flooding doesn't always occur whenever there is a perigean spring tide. However, perigean spring tides combined with seasonal changes in the tide and mean sea level may cause minor coastal flooding in some low-lying areas, often referred to as &ldquohigh tide flooding&rdquo or &ldquonuisance flooding&rdquo. Major coastal flooding typically occurs in response to strong onshore winds and barometric pressure changes from a coastal storm. If a storm strikes during a perigean spring tide, flooding could be significantly worse than it otherwise would have been. In some instances, perigean spring tides have coincided with a shift in offshore ocean circulation patterns and large scale shifts in wind that have resulted in unexpected coastal flooding. It is expected that occurrences of minor high tide flooding at the times of perigean spring tides will increase even more as sea level rises relative to the land. NOAA&rsquos tide and tidal current predictions take into account astronomical considerations due to the position of the moon and the sun.

Observations of a perigee coincident with a full Moon - Astronomy

It was explained to me that the giant full moon of this last April 16th appeared so bright and large because the moon was the closest to the Earth that it ever has been or ever will be (within a thousands of years kind of span.) How is this possible?

The Moon's orbit around Earth isn't a perfect circle - it's actually fairly elliptical - about 5.5% eccentricity. This means there's a fairly large difference between the perigee (when the Moon is at the closest point in its orbit) and apogee (when the Moon is at its farthest). This means that the Earth-Moon distance varies by about 13,000 miles either direction of the average distance. So if the full moon occurs at or near perigee, it appears noticeably larger in the sky than if it occurs at apogee, and it also it is brighter, because the amount of light received by the Earth from the Moon depends not only upon the amount of light the Moon gives off, but also how far the Earth is from the Moon. The farther the Moon, the smaller the fraction of the Moon's light that reaches Earth. I should add, however, that while this is a significant effect, all full moons are large and bright, so it's difficult to tell the difference without being able to look at a perigee and apogee full moon side by side. This year, the lunar perigee occurred only hours from the full moon on April 16th. It was the closest full moon of the year, but not the closest the Moon has been to the Earth in recent times. The nearest perigee recently was in 1912. For a much more detailed explanation, check out this site - it even has a link to a perigee and apogee calculator so if you want to observe this phenomenon you'll know when to take a look!

What other factors affect the brightness of the full moon?

There are several other factors that affect the brightness of the full moon. When the Earth (and therefore the Moon) is at its perihelion, the closest point in its orbit to the Sun, the sunlight that reflects off the Moon is slightly more intense, causing the full moon's brightness to increase by about 4%, which is imperceptible by the human eye.

The brightness of any object, including the moon, in the sky increases with its height in the sky. When an object is directly overhead, its light strikes the ground at a right angle, and the intensity of light is the same as the intensity in the beam. However, when an object is nearer to the horizon, its light strikes the ground at an angle, and the same amount of light is spread out over a larger area. Therefore, less light per unit area reaches the ground from an object near the horizon. Also, the closer the moon is to the horizon, the more atmosphere the light must travel through to reach the observer. This means that more of the moon's light is absorbed or scattered by the atmosphere. The height of the moon in the sky results from a combination of the latitude you are observing from and the declination of the moon.

When the moon is closer to opposition, that is, the point exactly opposite the Sun (at which point there is a lunar eclipse because the Sun's light is blocked by the Earth and does not reach the Moon), it is brighter. This is called the opposition effect. It is believed to be caused mainly by shadow hiding. The closer the moon is to opposition, the smaller the shadows cast by objects on its surface, and the brighter it appears. For more information on the opposition effect, check out this website.

Finally, atmospheric conditions have a great effect on the brightness of the full moon. The full moon on a clear night will be much brighter than if there is a lot of dust, smog or clouds.

About the Author

Cathy Jordan

Cathy got her Bachelors degree from Cornell in May 2003 and her Masters of Education in May 2005. She did research studying the wind patterns on Jupiter while at Cornell. She is now an 8th grade Earth Sciences teacher in Natick, MA.


The term supermoon is attributed to astrologer Richard Nolle while reading “Strategic Role Of Perigean Spring Tides in Nautical History and Coastal Flooding” published in 1976 by NOAA Hydrologist Fergus Wood. [5] [6] [7] In practice, there is no official or even consistent definition of how near perigee the full Moon must occur to receive the supermoon label, and new moons rarely receive a supermoon label.

Nolle Edit

Nolle described the concept in a 1979 edition of Dell Horoscope including both full and new moons, but has never outlined why he chose 90% nor has provided a definitive formula for determining if a given full or new moon is "super". The basic 1979 definition read: [2]

. a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit (perigee). In short, Earth, Moon and Sun are all in a line, with Moon in its nearest approach to Earth.

Nolle amended his definition in 2000 specifying the distance of a given full or new moon be judged against 90% of the mean distance of perigees. Nolle (incorrectly) referenced

A SuperMoon is a perigee-syzygy, a new or full moon (syzygy) which occurs when the Moon is at 90% or greater of its mean closest approach to Earth (perigee).

In 2011, Nolle added apogees to consideration explaining that he based calculations on 90% of the difference in lunar apsis extremes for the solar year. EarthSky analyzed Nolle's tables and described the updated definition as a full or new moon is considered a supermoon if l d s ≤ l d p + 0.1 ∗ ( l d a − l d p ) leq ld_


)> where l d s > is the lunar distance at syzygy, l d a is the lunar distance at apogee, and l d p > is the lunar distance at perigee. Nolle based those the mean apsis extremes referencing (incorrectly) the Wikipedia article on the subject arriving at: [10] [11]

any lunation closer than 368,630 km. = SuperMoon.

Nolle also added the concept of extreme supermoon in 2000 describing the concept as any new or full moons that are at "100% or greater of the mean perigee". [9]

Espenak Edit

The term perigee-syzygy or perigee full/new moon is preferred in the scientific community. [12] Perigee is the point at which the Moon is closest in its orbit to the Earth, and syzygy is when the Earth, the Moon and the Sun are aligned, which happens at every full or new moon. Astrophysicist Fred Espenak uses Nolle's definition but preferring the label of full Moon at perigee on full moons occurring "within 90% of its closest approach to Earth in a given orbit" over Nolle's calculations based on the closest of all orbits during the solar year. [13] Wood used the definition of a full or new moon occurring within 24 hours of perigee and also used the label perigee-syzygy. [7]

Other definitions Edit

Sky and Telescope magazine chose a definition of 223,000 miles (358,884 km). [14] prefers a definition of 360,000 kilometres (223,694 mi). [15]

EarthSky uses Nolle's definition comparing their calculations to tables published by Nolle in 2000. [10] [9]

Wood also coined the less used term proxigee where perigee and the full or new moon are separated by 10 hours or less. [7]

Of the possible 12 or 13 full (or new) moons each year, usually three or four may be classified as supermoons, as commonly defined.

The most recent full supermoon occurred on May 26, 2021, and the next one will be on June 24, 2021. [13]

The supermoon of November 14, 2016 was the closest full occurrence since January 26, 1948 and will not be surpassed until November 25, 2034. [16]

The closest full supermoon of the 21st century will occur on December 6, 2052. [17]

The oscillating nature of the distance to the full or new moon is due to the difference between the synodic and anomalistic months. [13] The period of this oscillation is about 14 synodic months, which is close to 15 anomalistic months. So every 14 lunations there is a Full Moon nearest to perigee.

Occasionally, a supermoon coincides with a total lunar eclipse. The most recent occurrence of this was in May 2021, and the next occurrence will be in May 2022. [13]

A full moon at perigee appears roughly 14% larger in diameter than at apogee. [18] Many observers insist that the moon looks bigger to them. This is likely due to observations shortly after sunset when the moon is near the horizon and the moon illusion is at its most apparent. [19]

While the moon's surface luminance remains the same, because it is closer to the earth the illuminance is about 30% brighter than at its farthest point, or apogee. This is due to the inverse square law of light which changes the amount of light received on earth in inverse proportion to the distance from the moon. [20] A supermoon directly overhead could provide up to 0.36 lux. [21]

Claims that supermoons can cause natural disasters, and the claim of Nolle that supermoons cause "geophysical stress", have been refuted by scientists. [2] [22] [23] [24]

Despite lack of scientific evidence, there has been media speculation that natural disasters, such as the 2011 Tōhoku earthquake and tsunami and the 2004 Indian Ocean earthquake and tsunami, are causally linked with the 1–2 week period surrounding a supermoon. [25] A large, 7.5 magnitude earthquake centred 15 km north-east of Culverden, New Zealand at 00:03 NZDT on November 14, 2016, also coincided with a supermoon. [26] [27] Tehran earthquake on May 8, 2020, also coincided with a supermoon.

Scientists have confirmed that the combined effect of the Sun and Moon on the Earth's oceans, the tide, [28] is greatest when the Moon is either new or full. [29] and that during lunar perigee, the tidal force is somewhat stronger, [30] resulting in perigean spring tides. However, even at its most powerful, this force is still relatively weak, [31] causing tidal differences of inches at most. [32]

Total lunar eclipses which fall on supermoon and micromoon days are relatively rare. In the 21st century, there are 87 total lunar eclipses, of which 28 are supermoons and 6 are micromoons. Almost all total lunar eclipses in Lunar Saros 129 are micromoon eclipses. An example of a supermoon lunar eclipse is the September 2015 lunar eclipse.

Annular solar eclipses occur when the Moon's apparent diameter is smaller than the Sun's. Almost all annular solar eclipses between 1880 and 2060 in Solar Saros 144 and almost all annular solar eclipses between 1940 and 2120 in Solar Saros 128 are micromoon annular solar eclipses. [33]

Perigee Full Moon

Honestly, I didn’t spend much time thinking about this “Supermoon”. Even though it would be a little unusual in that it was closest and biggest since 1948, the truth is, these differences are small. Supermoons, or perigee full moons as the scientists call them, are not that rare. This year had three. Hyping a “Supermoon” is mostly a media thing.

But the forecast was for clear, and it was a good excuse to get out of work, so I figured what the heck, I’d give it a try. That’s the nice thing about being self-employed.

The challenge is always to find something interesting… a great picture of just the Moon is ok, and certainly nothing to be sneezed at, but if you can combine it with something different, then you really have a picture.

I had spotted what I thought was a weather vane on top of a building in the town where I live and had filed that information away thinking maybe one day it might make a picture with the Sun or Moon behind it, if something interesting was going on with then. So I looked it up on Google Maps and used Google Compass and plotted the azimuth and elevation in my planetarium program and went out very early on the morning of the 13th to see if I could find a place to shoot it when it lined up. You have to consider not only the angle where it aligns, but the relative size of the weather vane in relation to the Moon at whatever focal length you intend to shoot it at. Since the weather vane is relatively small, I’d have to be relatively close to it, but to make the Moon big in the frame, I would need a long focal length, which meant I would need to be farther away from the weather vane. The problem was finding a place to shoot it from as buildings and trees quickly get in the way.

So I went out at 3:30am on the 13th and found a spot to shoot it with the AT65EDQ and 2x Barlow, which gave me 840mm of focal length. I just managed to get if framed before it went behind a tree, but the problem was, the weather vane was out of focus because it was much closer than the infinity focus for the Moon. So I used a technique called “focus stacking” where you combine one frame that is focused on the Moon with one that is focused on the weather vane.

It turned out that it that it wasn’t a weather vane at all. It was a compass pointing out the cardinal directions.

And did I mention that when I went out that morning, I thought that was the actual day of full moon? But, it turns out that I was a day early! The full moon was actually the next morning. All I can say is my sleep patterns have been really messed up and I must not have been thinking clearly. Doh!

Amazingly, the forecast was for clear the next day too. So I thought about going back to shoot the full moon with the compass vane (I’m not sure what it is actually called). But I thought a little more about what else would make a good picture and remembered a nice angle on the Philadelphia skyline where the Sun and Moon will sometimes align when they set. But for a particular phase of the Moon, or time of sunset or moonset, it only aligns with the skyline on one or two days of the year. What were the chances that it would align this morning? About 181 to 1.

I plotted the Moon’s altitude and azimuth at the time when it would be a degree or two above the horizon, and amazingly, it was within the window of the skyline! So I knew I had to try to make that picture if I got a break with the clouds, which the forecasts had coming.

Shooting the Full Moon Close-Up

I also wanted to try to shoot just a close-up of the Moon with my 5-inch refractor that night when it transited and was highest in the sky at 11:30pm. But I fell asleep with my wife, who goes to bed early so she can get up to go to exercise at the gym at 4 a.m. And I didn’t wake up until about 2 a.m. (I told you my sleep cycle has been messed up…)

So I thought what the heck, I would still try to shoot it since I should still have had time before it was over the skyline at about 6:15 a.m.

I went outside to set up the scope and the first thing I saw was the steam coming out of the rooftop vent for the furnace in the house. It was directly in a line for where the Moon was for where I set the scope up in the driveway. I knew that was not going to work because the heat coming out of there was going to totally trash the seeing. So I dragged all of my equipment down the block to set up between two houses so there would be no problems with heat rising off of them.

Got set up, and shot a bunch of frames. I wanted a high-resolution image of the full moon. For high-resolution planetary imaging, the preferred method is usually to shoot hundreds or thousands of frames and then let special software pick out the sharpest parts on the best frames and put them together into one really good frame. This is called “lucky imaging” because the software picks out the sharpest frames that were least affected by the atmospheric “seeing” or turbulence purely by luck.

The problem with lucky imaging for high-resolution is that you really need 1:1 pixel resolution off the sensor, and with video on most cameras, you don’t get that. It has to downsample the full 6,000 x 4,000 pixels to 1,920 x 1,080 pixels for the video. And this downsampling trashes the high-resolution detail.

For small planets, you can use sufficient magnification to get 1:1 pixel resolution off of a small area in the center of the sensor at a high framing rate with special software, but this won’t work for full frame, which is what was needed for the full moon.

The only way to get full resolution at full frame is to shoot single frames individually. And you have to use mirror lockup with a shutter delay to allow the vibration from the mirror slapping up to die down, which can affect sharpness.

I planned on using my new Nikon D5300 because it does not have a low-pass anti-aliasing filter and produces sharper images.

The seeing wasn’t that great, although it wasn’t terrible either, which was a little unusual for the winter, but a front had come through several days before and high pressure had been around, which makes for less turbulent air than just after a front passage.

But still, to deal with the seeing and use lucky imaging, you have to shoot a lot of frames. So I just sat there and patiently shot a couple of hundred frames, one at a time. I shot some at f/6.3 at prime focus, but with the refractor that is only about 819mm of focal length and the Moon didn’t fill the APS-C sized frame. So I put a 2x Barlow on it to increase the focal length to about 1,638mm, and the Moon very nicely filled the frame and I shot another 100 frames one-by-one.

The next day, I got around to processing the images. My plan was to process them in AutoStakkert!2, a great high-resolution planetary lucky imaging freeware program from Emil Kraaikamp.

I dumped the 100 JPEG images into the program and it calculated align points, but it stopped working when it tried to stack them. Each frame was about 68.7mb in 8 bits, and the 32-bit program just didn’t have enough memory, even though my computer has 32GB of ram and is 64-bit.

I contacted Emil and he suggested trying the latest beta version of the program, which was 64-bit. He said I might have problems with JPEGs though. So I processed all of the JPEGs into an AVI movie file with Planetary Image PreProcessor (PIPP), another great freeware program.

Then I opened the 2.34GB AVI file in AutoStakkert!2 and it did it’s thing. The processor use maxed out at 100 percent and 12GB of ram during the processing, but it worked!

Then I ran the stack through RegiStax and used wavelet sharpening to produce the image seen below.

Close-up shot of the perigee full moon “Supermoon”. Click on the image or link to see a larger version with more information.

Moon Set over Philadelphia

After shooting the Moon in close-up, by the time I got the scope put away and took the dog out to do his business, it was 5:15 a.m., and the Moon was going to be over the skyline at about 6:10 a.m. It wasn’t a far drive but I knew I would be cutting it close and it was probably stupid because you never know what might go wrong, like a flat tire. Or getting stopped by the police for speeding.

Anyway, I made it to the spot with about 10 minutes to spare, and found a couple of people there already with cameras! One was a friend of mine from our local astronomy club. And as we stood there shooting, about a half dozen more people showed up with cameras. But the funny thing was, none of them knew that the Moon only lined up with the skyline at that time and that phase one day a year! Any other full Moon in any other month, and it would not have set anywhere near the skyline!

I was worried that at 6:10am, when the Moon would be just about 2 degrees above the skyline, that the sky would be too bright, and it would wash out the view of the Moon because the Moon would be considerably dimmed by atmospheric extinction. Astronomical twilight started at 5:11 a.m., Nautical twilight at 5:43 a.m., Civil twilight at 6:15 a.m. and sunrise at 6:44 a.m. So the Moon would be in the right spot only one-half hour before sunrise. The question was, even if it was clear, would the Moon be visible at all…

Well, thanks to a very transparent air mass, the Moon was very bright all the way down to the horizon. In fact, when it was at about 3 to 2 degrees above the horizon, it was actually too bright for the correct exposure for the foreground of the skyline. At that elevation, I would have had to use two or more exposures for a high-dynamic range image to get both the Moon and the skyline correctly exposed.

By the time the Moon had gotten down to 1 degree altitude, the atmospheric extinction dimmed it sufficiently so that it was correctly exposed at the same exposure needed for the foreground of the skyline. And the buildings lit up red as they reflected the red sky in the east from the approaching dawn, which added a nice touch along with the reflections in the Cooper River.

“Supermoon” Moonset. Click on the image or link to see a larger version with more information.

In the end, a single 1/30th second exposure was used to produce the image immediately above. The raw file was opened in Adobe Camera Raw in Photoshop CC 2017 and the amazing dynamic range of the Nikon D5300 was put to use. At ISO 100, the camera has about 13 stops of dynamic range. This allowed me to adjust the exposure for the Moon while still holding detail in the darkest parts of the iamge.

This image, taken at 6:21 a.m. eastern daylight time, turned out to be right when the Moon was at perigee – it’s closest point in its orbit around the Earth, when it was largest in size. I originally thought it was two minutes before perigee, but my good friend Joe Steiber checked the U.S. Naval Observatory’s MICA software, and the moon was closest (356,508.987 km geocentric / selenocentric distance) from 11:20:48 to 11:21:30 UT (8:20:48 a.m. est to 8:21:30 a.m. est). Joe calculated the mid-time as 11:21:09 UT, or 6:21 am EST — the same time I took the picture.

All told, this “Supermoon” ended up being a super amount of work – about 3 full days of planning, shooting, and processing. But, I didn’t mind because I’m pretty happy with the images I got.

Comet Hale-Bopp 20 Years Ago

Click on the image to see a larger version and links to the pictures with more information on each.

It was 20 years ago today, Sgt. Pepper taught the band to play.

No, wait, that was 40 years ago!

I bet you are too young to remember that.

Anyway, I digress, lost in nostalgia.

It was 20 years ago on April 1, 1997 that Comet Hale-Bopp (C/1995 O1) reached perihelion.

I’ve put together a little layout with my best pix from that apparition.

Moon of Steel: Supermoon Returns

Oh, is it that time again? The time for everyone to overhype the “Supermoon”?

You’ve probably heard about this: on June 23, the full Moon will be the biggest and brightest of the year, so it’s called the Supermoon. Let me be clear: While this is technically true, you’d never notice the difference in size or brightness by eye. The full Moon will look pretty much like every other full Moon you’ve ever seen. Which is to say, big, bright, beautiful, and completely worth your time to outside and see! But Supermoon? Not so much.

Flying Around the Earth

Here’s the deal. The Moon orbits the Earth once every month or so. As it does so, the geometry—the angles between the Moon, Earth, and Sun—change, so we see a different part of the Moon lit every night. Once per orbit the Moon is roughly opposite the Sun in the sky, so the half facing the Earth is completely lit, and we say the Moon is full. This blog post I wrote (complete with awesome video) should help.

As it happens, the Moon orbits the Earth in an ellipse, not a circle, so the distance between us and the Moon changes all the time. When the Moon is closest to Earth in its orbit we call it perigee, and apogee when it’s farthest. These happen once per lunar orbit, of course, about 13 times per year each. This year, the average perigee distance is about 363,000 kilometers (225,000 miles), and the average apogee distance about 405,000 km (251,000 miles) [Note for math and astronomy pedants: astronomers measure distances using the centers of objects, so the distance to the surface of the Moon from the surface of the Earth is a bit smaller than this, by the sum of the radii of the two objects: about 8000 km.]

But those are averages the actual numbers month by month are all a bit different. The full Moon on June 23 will occur when the Moon is just a hair under 357,000 km (221,300 miles) away, the closest perigee of the year. The phase of the Moon and its distance from Earth are not connected in any way a full Moon can happen when the Moon is at apogee, perigee, or any point in between. It so happens this June 23 full Moon occurs just 20 minutes after perigee, so it really is about as close as it can get. That’s pretty nifty timing!

Up, Up, and Away!

So does this mean the Moon will look huger and brillianter in the sky? Not really. Last month, the full Moon happened when it was just over 358,000 kilometers away—only a little bit farther (by about 1 percent) than it will be this month. Even if you compared last month’s full Moon with this month’s “Supermoon” side-by-side you’d hardly notice it you’d never notice the difference just by going out one month to look, waiting a month, and looking again.

Heck, the difference between the two extremes of apogee and perigee is only about 40,000 km (25,000 miles)—about a 10-15 percent difference overall, making the Moon look 10-15 percent bigger at perigee. Even that wouldn’t be spotted by eye, especially with two weeks separating the two observations (not to mention the change in the phase of the Moon throwing your sense of scale off). Also, the Moon is a lot smaller in the sky than you think, so a small change is even harder to spot.

I have no doubt that people will still claim they can see the difference, and that the Moon looks much larger and brighter (and don’t confuse this with the Moon Illusion, where the Moon looks bigger on the horizon than when it’s high in the sky). I would bet a lot of money that’s just the power of suggestion if they hadn’t heard about the Supermoon in the first place they wouldn’t have even noticed any change. Because there’s no real change to notice!

This Supermoon silliness happens every year it started in 2011 and happened again last year. Some people even claimed the big 2011 earthquake in Japan was caused by the Supermoon, but that’s simply not true. The only real physical effect is on the tides, which are slightly stronger when the Moon is closer to the Earth, but that happens every month. We’ll have somewhat bigger tides on June 23 because the tides are larger when the Moon is full—its alignment opposite the Sun in the sky means their forces add together, slightly amplifying their combined effects—but it’s not that big a deal.

Artwork by Inga Nielsen, tweeted by PlanetPics. Tip o’ the lens cap to FakeAstroPix.

Contrary to popular belief, the Moon should ideally not be viewed at its full phase. During a full moon, rays of sunlight are hitting the visible portion of the Moon perpendicular to the surface. As a result, there is less surface detail visible during a full moon than during other phases (such as the quarter and crescent phases) when sunlight hits the Moon at a much shallower angle. The brightness of a full moon as compared to a phase where a smaller percentage of the surface is illuminated tends to wash out substantial amounts of detail and can actually leave an afterimage on an observer's eye that can persist for several minutes. First quarter (six to nine days past new moon) is generally considered the best time to observe the Moon for the average stargazer. Shadows and detail are most pronounced along the "terminator", the dividing line between the illuminated (day side) and dark (night side) of the Moon.

Naked eye Edit

Generally, the Moon can be viewed even with the naked eye, however it may be more enjoyable with optical instruments. The primary lunar surface features detectable to the naked eye are the lunar maria or "seas", large basaltic plains which form imaginary figures as the traditional "Moon Rabbit" or familiar "Man in the Moon". The maria cover about 35% of the surface. The contrast between the less reflective dark gray maria and the more reflective gray/white lunar highlands is easily visible without optical aid. Under good viewing conditions, those with keen eyesight may also be able to see some of the following features:

  1. Bright region around Copernicus
  2. Bright region around Kepler region region region
  3. Faintly shaded area near Sacrobosco
  4. Dark spot at foot of Mons Huygens

Another interesting phenomenon visible with the naked eye is Earthshine. Best visible shortly before or after new a moon (during the waning and waxing crescent phases respectively), Earthshine is the faint glow of the non-illuminated (night) side of the Moon caused by sunlight reflecting off the surface of Earth (which would appear nearly full to an observer situated on the Moon at this time) and onto the night side of the Moon. By the time the Moon reaches first its quarter however, the sunlight illuminated portion of the Moon becomes far too bright for Earthshine to be seen with the naked eye, however it can still be observed telescopically.

Binoculars and telephoto camera lenses Edit

Binoculars are commonly used by those just beginning to observe the Moon, and many experienced amateur astronomers prefer the view through binoculars over that through higher-power telescopes due to the larger field of view. Their high level of portability makes them the simplest device used to see more detail on the lunar surface than what is visible to the naked eye.

The primary disadvantage of binoculars is that they cannot be held as steadily unless one utilizes a commercial or homemade binocular tripod. The recent introduction of image-stabilized binoculars has changed this to some extent however, cost is still an issue.

A 10× pair of binoculars will magnify the Moon approximately as much as a 200mm camera lens can. The photos below were shot with a 200mm lens. The first photo was taken on 13 November 2016 at 6:20pm PST, observing the full Moon just hours before it would officially become the largest supermoon since 1948. The second photo was shot 24 hours later, and the contrast was enhanced to bring out details such as mountainous terrain. The next supermoon will not occur this large until the year 2034.

Pre-supermoon on November 13, 2016 ( 2016-11-13 ) , at 6:20 pm PST, just a few hours before it would be officially called the "Supermoon of 2016"

5 keys to enjoying the closest supermoon

On November 14, 2016, the moon will be closer to Earth than it’s been since January 26, 1948. It’ll be a full moon and a supermoon. The moon won’t come this close again until November 25, 2034. That makes upcoming full moon the closest and largest supermoon in a period of 86 years! Here are five things you need to know.

Mother and sons watch a 2013 supermoon rise through cloud cover at Tempe Town Lake in Arizona. Photo via Kathleen Kingma. Read more about this image.

Moon equally awesome on November 13 and 14. Here’s the first and most important thing you need to know. Many articles we’ve seen about the coming supermoon say to look for it on November 14. But – for many of us, especially those of us in the Americas – the moon will be just as big and bright (if not bigger and brighter) on November 13.

That’s because the moon will reach both the crest of its full phase – and its closest point for the month (perigee) – very early in the day on November 14 according to clocks in the Americas.

Perigee comes at 11:23 UTC (6:23 a.m. ET) on November 14.

Full moon crests two-and-a-half hours later at 13:52 UTC (8:52 a.m. ET) on November 14.

So – for all of us in the Americas – the moon is closest to being full and closest to Earth on the morning of November 14, not the evening. That means that – for all U.S. time zones, including Alaska and Hawaii – the supermoon falls closer to the night of November 13 than November 14. That’s especially true if you are a morning person and plan to observe the supermoon before dawn.

Don’t stress too much about this. The moon will be big and bright both nights! And both nights will be an awesome time to moon-gaze or take photographs.

At right, the August 29, 2015 supermoon. At left, the March 5, 2015 ‘micro-moon’ – smallest full moon of the year. Photos by Peter Lowenstein in Mutare, Zimbabwe.

Are supermoons hype? No. Here’s what to look for The term supermoon is relatively new (read their history here). Before we called them supermoons, we in astronomy called them perigean full moons. Catchy, eh? Well … not so much. Most people ignored these moons until the term supermoon came along.

What’s special about a supermoon? Finely tuned instruments – or composite images – show that a supermoon is indeed closer to Earth and thus bigger than an ordinary full moon.

But most of us can’t detect that difference, using just our eyes. Experienced observers, meanwhile, do sometimes say they can detect a size difference between ordinary full moons and a supermoon.

So, if the rest of us can’t see that a supermoon actually appears larger in our sky, why do we get so excited about supermoons? Here are two things to notice about the November 13-14 supermoon.

First, for all of us, the brightness of the moon will increase noticeably around supermoon-time. All full moons are bright, but supermoons are substantially and noticeably brighter than ordinary full moons. So … notice the brightness, not the bigness, of the moon on November 13 and 14!

Second, the moon’s gravity affects earthly tides, and a supermoon – full moon closest to Earth – pulls harder on Earth’s oceans than an ordinary full moon. That’s why supermoons create higher-than usual tides. Read on …

Deik Haigler Photography wrote of the September 2015 supermoon tide: “We almost got stuck out the Marsh Boardwalk when this King High Tide came in over the boardwalk…”

Supermoons can create super tides. Are supermoons hype? Just ask the oceans! All full moons bring larger-than-usual tides, called spring tides or, in some places, king tides.

Supermoons bring the highest, and lowest, tides of all.

If you live along a coastline, watch for high tides caused by the November 14 supermoon for a period of several days after November 14. These tides tend to follow the date of full moon by a day or two.

Will the high tides cause flooding? Probably not, unless a strong weather system moves into the coastline where you are. That was the case with the high supermoon tides of September, 2015. That supermoon – combined with an 18.6-year lunar cycle, and a tropical storm – caused high tides and some flooding on both sides of the Atlantic.

So keep an eye on the weather around November 14, if you live along the coast. Storms do have a large potential to accentuate high spring tides, especially those caused by supermoons.

Moon is closest to Earth at perigee and farthest away at apogee. When the full moon aligns with perigee – as it does on November 14, 2016 – it’s a perigee full moon or supermoon. By the way, the moon’s orbit is much closer to being a circle than this diagram suggests! Image via NASA.

Closest moon nearly always a full moon. We wondered … is it the closest moon (in general) since 1948, or the closest full moon? Turns out those two tend to be one and the same.

Because of gravity, and the intriguing interplay of the sun, Earth and moon (and, to a lesser extent, the planets), the closest perigee of any given year is often the one that aligns most closely with full moon.

For the moon to appear full, the sun, Earth and moon need to be aligned, with Earth in the middle. During that particular alignment, the tidal pull of the sun and moon combine to create wide-ranging spring tides. And full moons at perigee create even wider-ranging perigean spring tides.

The diagram below helps to explain why the perigee full moon comes especially close to Earth. Take a careful look. Explanation below.

Image via Bedford Astronomy Club.

Ready to get technical? Read on!

On the diagram above, the line connecting lunar perigee with lunar apogee defines the moon’s major axis (the longest axis of an ellipse).

When the moon’s major axis (apogee-perigee line) points sunward (A & C on the diagram), the eccentricity (flatness) of the moon’s orbit is increased to a maximum. A greater eccentricity lessens the perigee distance, while increasing the apogee distance.

At A in the diagram, it’s a perigee new moon (supermoon) and an apogee full moon (micro-moon).

Then 3.5 lunar months (some 103 days) later, at B in the diagram, the major axis is at a right angle to the sun-Earth line, so the eccentricity is minimal. At such times, the moon’s orbit is closest to circular. It’s a more distant perigee and closer apogee, with the first quarter and last quarter moons more or less aligning with apogee and perigee.

Then 7 lunar months (206 days) later, the major axis again points sunward. Once again, the eccentricity of the moon’s orbit is increased to a maximum, lessening the perigee distance yet increasing the apogee distance. This time around, it’s a full moon perigee and new moon apogee. See C in the diagram.

Dates for closest/farthest new/full moons in 2016:

April 7: closest new moon
April 22: farthest full moon

Seven lunar months later:

October 30: farthest new moon
November 14: closest full moon

Supermoons happen in cycles. Okay, so by now – if you’re a regular EarthSky reader – you’ve figured out that everything in the sky happens in cycles. The supermoon is no exception.

Closest full moons tend to recur in cycles of 14 lunar (synodic) months, because 14 lunar months almost exactly equal 15 returns to perigee (moon’s closest point to Earth).

A lunar month refers to the time period between successive full moons, a mean period of 29.53059 days. An anomalistic month refers to successive returns to perigee, a period of 27.55455 days. Hence:

14 lunar months x 29.53059 days = 413.428 days
15 anomalistic months x 27.55455 days = 413.318 days

This 413-day time period is equal to about 1 year, 1 month, and 18 days.

The full moon and perigee will realign again on January 2, 2018, because the 14th full moon after the November 14, 2016 full moon will fall on that date.

Moon closest to Earth

Looking further into the future, the perigee full moon will come closer than 356,500 kilometers for the first time in the 21st century (2001-2100) on November 25, 2034 (356,446 km). The closest full moon of the 21st century will fall on December 6, 2052 (356,425 km).

For the moon to come closer than 356,400 kilometers (221,457 miles) is quite a feat. In fact, this won’t happen at all in the 21st century (2001-2100) or the 22nd century (2101-2200). The last time the full moon perigee swung this close to Earth was on January 14, 1930 (356,397 km), and the next time won’t be till January 1, 2257 (356,371 km).

Supermoons happen every year. Supermoon and shipwreck – August 10, 2014 – by Damian McCudden.

Bottom line: Enjoy the November 14, 2016 supermoon. Don’t forget to watch for it on November 13 as well, especially if you live in the Americas. Two things to watch for: the moon’s brightness, and especially high tides in the days following the full moon.