Why Does The Moon Look So Big Tonight?

Look at the Moon. Have you ever noticed the Moon looks so big when it’s down on the horizon, but way smaller when it’s nearly overhead? What’s going on here? Turns out, you fell for the oldest trick in the book: the Moon Illusion.

Look at that Moon. It looks so big and full. Way bigger than it normally does. I wonder what’s going on to make it look so big? Maybe it’s closer and brighter? Maybe the atmosphere is distorting it like a lens? Or maybe, I’m just a human being, and I just fell for the oldest trick in the book: the Moon Illusion. Which really sounds more like a 80’s spy thriller novel than anything else. What I’m saying is, don’t believe your eyes.

The Moon is always the same size, and the distance varies by only a small amount during its orbit. As a result, the Moon is roughly the same size in the sky every night. Even though it looks huge on the horizon, it’s identical to when it’s directly overhead.

Don’t believe me? The Moon and your pinky fingernail when you hold your arm out at length, are about the same size. Next time the Moon’s in the sky, try it out, and you’ll see. Then try this out on one of those nights when the Moon just looks so big and fat. It’ll be it’s exactly the same size as it was before.

Look at this picture. Look at this collection of Moons, taken one after the other from Moonrise until the Moon is high in the sky. Exactly the same size! Every time! So what’s going on here?

The problem is up here, in my meat-thinky parts. For some reason, when the Moon is down on the horizon, we think it’s larger than when it’s directly overhead. But why? Bad news, we’re not actually sure yet. We’re still piling up the list of cognitive biases that make us think it’s a good idea to stay on an airplane that’s on fire or convince us to wait it out in our homes when there’s a tornado headed straight for us instead of evacuating like the nice people on the radio say.

How we perceive the moon's size may have much to do with what's around it. In this illustration, most of us seen the bottom moon as smaller, but they're both exactly the same size. Crazy, isn't it? Credit: NASA
How we perceive the moon’s size may have much to do with what’s around it. In this illustration, most of us seen the bottom moon as smaller, but they’re both exactly the same size. Crazy, isn’t it? Credit: NASA

One idea is that the Moon looks bigger on the horizon because it looks farther away. When we see stuff in the sky, like clouds, birds or airplanes, they seem tiny. But when we see the Moon, compared to closer objects on the horizon, like trees and buildings, our brain freaks out and decides that it’s actually larger.

Fun fact! It turns out our brain is really bad at knowing how big things actually are, and it’s easily tricked by the stuff around it. Here’s an optical illusion called the Ebbinghaus illusion. See those circles in the middle? They’re the same size in each example. But because of the other circles around them, our brain can’t deal. Normally buildings and trees are big. And yet they seem tiny compared to the Moon on the horizon.

I did say that it’s mostly the same distance, every night but the Moon actually does get bigger and smaller in the sky. It’s following an elliptical orbit around the Earth. At its closest point, the Moon gets about 363,000 km. And then at its furthest point, it’s about 405,000 km. So that is a bit of a difference, but seriously, you’d need a really good telescope to be able to tell, and it takes almost a month to make this journey from one end to the other.

Moon timelapse. Credit: Cory Schmitz
Moon timelapse. Credit: Cory Schmitz

Trust me, you can’t tell. Or you know what, you can tell, you’re right. It’s just me, and everyone else, for us regular mortals, our brains are fooled. So next time your friend mentions how huge the Moon looks, feel free to explain the cold hard facts to them. Let them know that their brain is lying to them, and how they’re easily deceived. Then laugh and mock them for their amusing little human frailties. Then, I suppose you might be looking for new friends… but you will have enlightened them to the way of their wrongness, and that’s a gift that keeps on giving.

Well, did you fall for this? Did you think the Moon looks huge on the horizon, or are you somehow immune to the Moon illusion? If so, tell us your secret in the comments below.

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Guide to Tonight’s Big Harvest Moon

Tonight, September 8, the Harvest Moon rises the color of a fall leaf and spills its light across deserts, forests, oceans and cities. The next night it rises only a half hour later. And the next, too. The short gap of time between successive moonrises gave farmers in the days before electricity extra light to harvest their crops, hence the name.

The Harvest Moon is the full moon that falls closest to the autumnal equinox, the beginning of northern autumn. As the moon orbits the Earth, it moves eastward about one fist held at arm’s length each night and rises about 50 minutes later. You can see its orbital travels for yourself by comparing the moon’s nightly position to a bright star or constellation. 

This full Moon is also a Proxigean or Perigee Full “Supermoon” (find out more about that here), which means the Moon is in a spot in its elliptical orbit where it is closer to Earth near the time it is full, making it look up to 15% larger than average full Moon.

Around the time of Harvest Moon, the full moon's path is tilted at a shallow angle to the eastern horizon making with successive moonrises only about a half hour apart instead of the usual 50 minutes. Source: Stellarium
Around the time of Harvest Moon, the full moon’s path is tilted at a shallow angle to the eastern horizon making with successive moonrises only about a half hour apart instead of the usual 50 minutes. Source: Stellarium

50 minutes is the usual gap between moonrises. But it can vary from 25 minutes to more than an hour depending upon the angle the moon’s path makes to the eastern horizon at rise time. In September that path runs above the horizon at a shallow angle. As the moon scoots eastward, it’s also moving northward this time of year.

This northward motion isn’t as obvious unless you watch the moon over the coming week. Then you’ll see it climb to the very top of its monthly path when it’s high overhead at dawn. The northward motion compensates for the eastward motion, keeping the September full moon’s path roughly parallel to the horizon with successive rise times only ~30 minutes apart.

The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 20+ minutes apart. Times are shown for the Duluth, Minn. region. Illustration: Bob King
The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 30+ minutes apart. Times are shown for the Duluth, Minn. region. Illustration: Bob King

Exactly the opposite happened 6 months earlier this spring, when the moon’s path met the horizon at a steep angle. While it traveled the identical distance each night then as now, its tilted path dunked it much farther below the horizon night to night. The spring full moon moves east and south towards its lowest point in the sky. Seen from the northern hemisphere, that southward travel adds in extra time for the moon to reach the horizon and rise each successive night.

If all this is a bit mind-bending, don’t sweat it. Click HERE to find when the moon rises for your town and find a spot with a great view of the eastern horizon. You’ll notice the moon is orange or red at moonrise because the many miles of thicker atmosphere you look through when you gaze along the horizon scatters the shorter bluer colors from moonlight, tinting it red just as it does the sun.

A series of photos of the full moon setting over Earth's limb taken by from space by NASA astronaut Don Pettit on April 16, 2003. Refraction causes a celestial object's light to be bent upwards, so it appears higher than it actually is. The bottom half of the moon, closer to the horizon, is refracted strongest and "pushed" upward into the top half, making it look squished. Credit: NASA
A series of photos of the full moon setting over Earth’s limb taken by from space by NASA astronaut Don Pettit on April 16, 2003. Refraction causes a celestial object’s light to be bent upwards, so it appears higher than it actually is. The bottom half of the moon, closer to the horizon, is refracted strongest and “pushed” upward into the top half, making it look squished. Credit: NASA

The moon will also appear squished due to atmospheric refraction. Air is densest right at the horizon and refracts or bends light more strongly than the air immediately above it. Air “lifts” the bottom of the moon – which is closer to the horizon – more than the top, squishing the two halves together into an egg or oval shape.

How we perceive the moon's size may have much to do with what's around it. In this illustration, most of us seen the bottom moon as smaller, but they're both exactly the same size. Crazy, isn't it? Credit: NASA
How we perceive the moon’s size may have much to do with what’s around it. In this illustration, most of us seen the bottom moon as smaller, but they’re both exactly the same size. Crazy, isn’t it? Credit: NASA

You may also be entranced Monday night by the Moon Illusion, where the full moon appears unnaturally large when near the horizon compared to when viewed higher up. No one has come up with a complete explanation for this intriguing aspect of our perception, but the link above offers some interesting hypotheses.

Can you see craters with your naked eye? Yes! Try tonight through Wednesday night. Plato is the trickiest. Credit: Bob King
Can you see craters with your naked eye? Yes! Try tonight through Wednesday night. Plato is the trickiest. Credit: Bob King

Finally, full moon is an ideal time to see several lunar craters with the naked eye. They’re not the biggest, but all, except Plato, are surrounded by bright rays of secondary impact craters that expand their size and provide good contrast against the darker lunar “seas”. Try with your eyes alone first, and if you have difficulty, use binoculars to get familiar with the landscape and then try again with your unaided eyes.

In contrast to the other craters, Plato is dark against a bright landscape. It’s a true challenge – I’ve tried for years but still haven’t convinced myself of seeing it. The others are easier than you’d think. Good luck and clear skies!

If you don’t have clear skies, Slooh will broadcast the “Super Harvest Moon” live from the Institute of Astrophysics of the Canary Islands, off the coast of Africa. Slooh’s live coverage will begin at 6:30 PM PDT / 9:30 PM EDT /01:30 UTC (8/9) – International times here. Slooh hosts are Geoff Fox and Slooh astronomer Bob Berman. Viewers can ask questions during the show by using hashtag #Sloohsupermoon. Watch below:

What is a Super Moon?

It’s a bird, it’s a plane it’s…

OK, it’s a bad gag, I know. But the movie Man of Steel isn’t the only thing that’s “super” about June this year. The closest full Moon of 2013 occurs on June 23, when it will be 356,991 kilometres from Earth, within 600 kilometres of its closest possible approach. When the Moon is closest to Earth in its orbit, it also appears just a bit larger in the sky. But that’s if you’re really paying attention, however!

Some claims circulating on the Internet tend to exaggerate how large the Moon will actually appear. And as for the assertions that the Moon will look bright purple or blue on June 23, that’s just not true. As seems to happen every year, the term “supermoon” has once again reared its (ugly?) head across ye ole Internet. Hey, it’s a teachable moment, a good time to look at where the term came from, and examine the wonderful and wacky motion of our Moon.

I’ll let you in on a small secret. Most astronomers, both of the professional and backyard variety, dislike the informal term “supermoon”. It arose in astrology circles over the past few decades, and like the term “Blue Moon” seems to have found new life on the Internet.  A better term from the annuals of astronomy for the near-coincidence of the closest approach of the Full Moon would be Perigee Full Moon. And if you really want to be archaic, Proxigean Moon is also acceptable.

On June 23, 2013, the Moon will be full at 7:32 AM EDT/ 11:32 UT, only 20 minutes after it reaches perigee, or its closest point to Earth in its orbit.

You can see the change in apparent size of the Moon (along with a rocking motion of the Moon known as nutation and libration) in this video from the Goddard Space Flight Center’s Scientific Visualization Studio. You can also see full animations for Moon phases and libration for 2013 from the northern hemisphere and southern hemisphere.

And all perigees are not created equal, either. Remember, a Full Moon is an instant in time when the Moon’s longitude along the ecliptic is equal to 180 degrees. Thus, the Full Moon rises (unless you’re reading this from high polar latitudes!) opposite as the Sun sets. Perigee also oscillates over a value of just over 2 Earth radii (14,000 km) from 356,400 to 370,400 km. And while that seems like a lot, remember that the average distance to the Moon is about 60 earth radii, or 385,000 km distant.

Astronomers yearn for kryptonite for the supermoon. The Moon passes nearly as close every 27.55 days, which is the time that it takes to go from one perigee to another, known as an anomalistic month. This is not quite two days shorter than the more familiar synodic month of 29.53 days, the amount of time it takes the Moon to return to similar phase (i.e. New to New, Full to Full, etc).

This offset may not sound like much, but 2 days can add up. Thus, in six months time, we’ll have perigee near New phase and the smallest apogee Full Moon of the year, falling in 2013 on December 19th. Think of the synodic and anomalistic periods like a set of interlocking waves, cycling and syncing every 6-7 months.

You can even see this effect looking a table of supermoons for the next decade;

Super Moons for the Remainder of the Decade 2013-2020.

Year

Date

Perigee Time

Perigee Distance

Time from Full

Notes

2013

June 23

11:11UT

356,989km

< 1 hour

2013

July 21

20:28UT

358,401km

-21 hours

2014

July 13

8:28UT

358,285km

+21 hours

2014

August 10

17:44UT

356,896km

< 1 hour

2014

September 8

3:30UT

358,387km

-22 hours

2015

August 30

15:25UT

358,288km

         +20 hours

2015

September 28

1:47UT

356,876km

-1 hour

Eclipse

2015

October 26

13:00UT

358,463km

-23 hours

2016

October 16

23:37UT

357,859km

+19 hours

Farthest

2016

November 14

11:24UT

356,511km

-2 hours

Closest

2017

December 4

8:43UT

357,495km

+16 hours

2018

January 1

21:56UT

356,565km

-4 hours

2019

January 21

19:59UT

357,344km

+14 hours

Eclipse

2019

February 19

9:07UT

356,761km

-6 hours

2020

March 10

6:34UT

357,122km

+12 hours

2020

April 7

18:10UT

356,908km

-8 hours

Sources: The fourmilab Lunar Perigee & Apogee Calculator & NASA’s Eclipse Website 2011-2020.Note: For the sake of this discussion, a supermoon is defined here as a Full Moon occurring within 24 hours of perigee. Other (often arbitrary) definitions exist!

Note that the supermoon slowly slides through our modern Gregorian calendar by roughly a month a year.

In fact, the line of apsides (an imaginary line drawn bisecting the Moon’s orbit from perigee to apogee) completes one revolution every 8.85 years. Thus, in 2022, the supermoon will once again occur in the June-July timeframe.

To understand why this is, we have to look at another unique feature of the Moon’s orbit. Unlike most satellites, the Moon’s orbit isn’t fixed in relation to its primaries’ (in this case the Earth’s) equator. Earth rotational pole is tilted 23.4 degrees in relation to the plane of its orbit (known as the ecliptic), and the Moon’s orbit is set at an inclination of 5.1 degrees relative to the ecliptic. In this sense, the Earth-Moon system behaves like a binary planet, revolving around a fixed barycenter.

The two points where the Moon’s path intersects the ecliptic are known as the ascending and descending nodes. These move around the ecliptic as well, lining up (known as a syzygy) during two seasons a year to cause lunar and solar eclipses.

The complex motion of the Moon, depicting the precession of the nodes versus the average movement of the line of apsides. (Credit: Geologician, Homunculus 2. Wikimedia Commons graphic  under a Creative Common Attribution 3.0 Unported license).
The complex motion of the Moon, depicting the movement of the nodes versus the average movement of the line of apsides. (Credit: Geologician, Homunculus 2. Wikimedia Commons graphic under a Creative Common Attribution 3.0 Unported license).

But our friend the line of apsides is being dragged backwards relative to the motion of the nodes, largely by the influence of our Sun. Not only does this cause the supermoons to shift through the calendar, but the Moon can also ride ‘high’ with a declination of around +/-28 degrees relative to the celestial equator once every 19 years, as happened in 2006 and will occur again in 2025.

Falling only two days after the solstice, this month’s supermoon is also near where the Sun will be in December and thus will also be the most southerly Full Moon of 2013. Visually, the Full Moon only varies 14% in apparent diameter from 34.1’ (perigee) to 29.3’ (apogee).

Can you see the difference? A side by side comparison of the perigee and apogee Moon. (Credit: Inconstant Moon).
Can you see the difference? A side by side comparison of the perigee and apogee Moon. (Credit: Inconstant Moon).

A fun experiment is to photograph the perigee Moon this month and then take an image with the same setup six months later when the Full Moon is near apogee. Another feat of visual athletics would be to attempt to visually judge the Full Moons throughout a given year. Which one do you think is largest & smallest? Can you discern the difference with the naked eye? Of course, you’d also have to somehow manage to insulate yourself from all the supermoon hype!

A comparison of the rising Moon (left) & the Full Moon high in the sky... as you can see, atmospheric refraction actually tends to "shrink" the apparent size of a rising Moon! (Credit:
A comparison of the rising Moon (left) & the Full Moon high in the sky… as you can see, atmospheric refraction actually tends to “shrink” the apparent size of a rising Moon! (Credit & Copyright: Richard Fleet (@dewbow) The Moon Illusion). 

Many folks also fall prey to the rising “Moon Illusion.” The Moon isn’t visually any bigger on the horizon than overhead. In fact, you’re about one Earth radii closer to the Moon when it’s at the zenith than on the horizon. This phenomenon is a psychological variant of the Ponzo illusion.

The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Note the size difference. Image Credit: Marco Langbroek, the Netherlands, via Wikimedia Commons.
The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Note the size difference. Image Credit: Marco Langbroek, the Netherlands, via Wikimedia Commons.

Here are some of the things that even a supermoon can’t do, but we’ve actually heard claims for:

–      Be physically larger. You’re just seeing the regular-sized Moon, a tiny bit closer.

–      Cause Earthquakes. Yes, we can expect higher-than-normal Proxigean ocean tides, and there are measurable land tides that are influenced by the Moon, but no discernible link between the Moon and earthquakes exists. And yes, we know of the 2003 Taiwanese study that suggested a weak statistical correlation. And predicting an Earthquake after it has occurred, (as happened after the 2011 New Zealand quake) isn’t really forecasting, but a skeptical fallacy known as retrofitting.

–      Influence human behavior. Well, maybe the 2013 Full Moon will make some deep sky imagers pack it in on Sunday night. Lunar lore is full of such anecdotes as more babies are born on Full Moon nights, crime increases, etc. This is an example the gambler’s fallacy, a matter of counting the hits but not the misses. There’s even an old wives tale that pregnancy can be induced by sleeping in the light of a Full Moon. Yes, we too can think of more likely explanations…

–      Spark a zombie apocalypse. Any would-be zombies sighted (Rob Zombie included) during the supermoon are merely coincidental.

Do get out and enjoy the extra illumination provided by this and any other Full Moon, super or otherwise. Also, be thankful that we’ve got a large nearby satellite to give our species a great lesson in celestial mechanics 101!