Tag: perigee

Followup: Supereclipse

By Phil Plait | May 19, 2012 7:00 am

I wrote earlier about the annular eclipse happening this coming Sunday. It’s a solar eclipse, with the Moon blocking the Sun, but because the Moon is at apogee — the point in its orbit farthest from Earth — the Moon appears smaller in the sky, so it doesn’t completely block the Sun. We’re left with a ring of solar surface surrounding the Moon, the so-called Ring of Fire.

I got a couple of people asking me why this eclipse is happening at lunar apogee when we just had a "Supermoon", when the Moon was full at perigee (when it’s closest to Earth in its orbit). This is a good question! It’s not a coincidence. In fact, it must happen this way! Here’s why.

First, here’s a drawing of the Moon’s orbit, courtesy NASA:

The Moon orbits the Earth in an ellipse, so sometimes it’s closer to us, and sometimes farther. The ellipticity is exaggerated in the drawing; it’s actually about a 10% difference in distance between apogee and perigee. The Moon orbits the Earth once every 27.3 days, so it takes about 13.7 days for it to go from apogee to perigee — a little less than two weeks.

This is different than the phase of the Moon, which is how much of the Moon we see lit by the Sun. When the moon is between us and the Sun, it’s new: we only see the unlit side. When it’s opposite the Sun in the sky — when the Earth is between the two — the side of the Moon we see is lit, so we say it’s full. There are approximately 8 billion web pages describing how this works; here’s one I wrote. The time it takes to go from full Moon to full Moon is 29.5 days. That means to go from full Moon to the next new Moon takes half that time, or about 14.7 days — a little more than two weeks.

We can only get a solar eclipse when the Moon is between us and the Sun. This happens when the Moon is new (I’ll note in passing that it doesn’t happen every time the Moon is new, because the orbit of the Moon doesn’t align exactly with the Earth’s orbit around the Sun).

The phases of the Moon don’t line up perfectly with its position in the orbit because of the two different periods: 27.3 days to go around the Earth, but 29.5 days to go from full to full again (this video might help you). So sometimes full Moon happens at perigee, sometimes at apogee, and most of the time sometime in between.

Now let’s put this all together! The Supermoon is when the Moon is full and at perigee, right? That’s what happened on May 5th. On Sunday, a bit more than two weeks will have elapsed since then. That means the Moon will have moved halfway around its orbit — it actually reaches apogee on Saturday May 19th. But the phase has been changing, so it’s new on May 20, and it so happens that things have aligned for it to eclipse the Sun.

Since this happens the day after apogee, the Moon is farther away than usual, and from Earth it looks smaller. BOOM. Annular eclipse.

I think the confusion stems from folks not knowing the Moon orbits the Earth once per month on an ellipse, so it goes from perigee to apogee in two weeks. Once you get that, hopefully the rest of this makes more sense.

And because why not, I’ll leave you with this video showing the phase of the Moon as well as its apparent size in the sky as they change over the course of the year. If you want a detailed explanation of what you’re seeing, here ya go.

Enjoy the eclipse! And make sure if you watch it, you do so safely.

Image credits: NASA; Sancho Panza on Flickr.


Related Posts:

Ring of fire eclipse on May 20
Solar eclipse, from space!
Newest of new moons
Record breaker: newest new Moon spotted!
What does a lunar eclipse look like from the Moon?

CATEGORIZED UNDER: Astronomy, Cool stuff

The Supermoon stuff? AGAIN?

By Phil Plait | May 2, 2012 10:37 am

Sigh.

You may’ve seen some folks writing about this weekend’s so-called Supermoon. I suppose I’m not surprised, but it’s still irritating. Why? Because it’s just hype (and to get this out of the way immediately, will have no real effect on the Earth, either). Here’s the scoop.

This weekend, on the night of May 5/6, the Moon will be full. This happens every 29 days or so when the Moon is opposite the Sun in the sky, and we see its face fully illuminated.

As it happens, the Moon’s orbit is elliptical, and so sometimes the Moon is a bit closer to the Earth than other times. Every now and again the Moon is full when it’s also closest to Earth — the point in its orbit called perigee. May 5th is one of those times.

What does this mean? Well, it means the Moon is closer, so it will appear a bit bigger and brighter than usual. But here’s the thing: you’d never know. Seriously, to the eye it’ll look exactly the same as it always does when it’s full. The Moon is actually pretty small in the sky — if you don’t believe me, go outside tonight, find the Moon, and hold your thumb up at arm’s length next to it; it’ll easily cover the Moon entirely (my thumb is 2 – 3 times wider than the Moon). A small change in its size is something that’s really hard to see.

To be specific, according to Fourmilab, the Moon will be 356,953 kilometers from Earth when it’s full. However, last month, on April 7, when it was full it was about 358,313 km away. That’s a difference of 1400 km, less than 1%. So really, the size of the full Moon this weekend won’t be any different than it was last month, and no one was writing about it then. And to show I’m not being biased, take a look at when the Moon was full near apogee — the most distant point in its orbit. That’ll happen in late November of 2012, when it’ll be at a distance of 406,364 km. That’s still only a difference of less than 14%.

That’s a pretty small change, not enough to notice by eye. Read More

Distant full Moon tonight

By Phil Plait | October 11, 2011 6:18 pm

I almost missed this, but an email from astrophotographer Anthony Ayiomamitis (whose photo I feature below) reminded me: tonight’s full Moon occurs at apogee, the point in the Moon’s orbit where it is most distant from Earth. I actually wrote quite a bit about this last year, so I’ll repost the article below. Full Moon occurs officially tonight at 02:06 UTC (10:06 p.m. Eastern US time), so in a couple of hours as I write this. Apogee occurs about 9 hours later (October 12 at 11:44 UTC), when the Moon will be 406,176 km (252,286 miles) from the Earth. It was at perigee on September 28, when it was a mere 357,555 km (222,174 miles) from us… but make sure you read the footnote below!

And I’ll note: the difference in size between the Moon at closest and farthest approach isn’t something you’d probably never notice it by eye, especially since you can’t compare the two at the same time. The change is gradual, and the Moon is actually pretty small in the sky. But it’s still neat when you take a picture and compare them…


I’ve been posting a lot of extreme close-ups of the Moon, but sometimes you can learn something by taking a step back.

For example, I imagine if I went out in the street and asked people what shape the Moon’s orbit was, they’d say it was a circle (or, given recent poll results, they’d say it was Muslim). In fact, however, the Moon’s orbit is decidedly elliptical. When it’s closest to Earth — the point called perigee — it’s roughly 360,000 kilometers (223,000 miles) away*, and when it’s at its farthest point — apogee — it’s at a distance of about 405,000 km (251,000 miles).

That’s a difference of about 10% — not enough to tell by eye, but certainly enough to see in a picture… like this one, by the Greek amateur astronomer Anthony Ayiomamitis:

lunar-apogee-perigee-2010

[Click to emperigeenate.]

Amazing, isn’t it? The Moon is noticeably different! He took those images at full Moon, but seven months apart, when the Moon was at perigee (last January) and apogee (just a few days ago as I write this). It’s part of a project he does every year, and it’s pretty cool. He was able to get these images within a few moments of the exact times of apogee and perigee.

You might wonder how the Moon can be at apogee when it’s full one time, and perigee at another time it’s full. Read More

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

The proxigean, perigean Moon

By Phil Plait | March 19, 2011 8:49 pm

You know what? Super or not, the full Moon rising into a thin cloud bank is always going to be cool.

I just took this from the end of my driveway, with my camera on a cheap tripod and a few seconds exposure. No tricks, no Photoshopping (except to crop it a bit). You can almost hear the werewolf howling, can’t you?

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
MORE ABOUT: Moon, perigee

No, the "supermoon" didn't cause the Japanese earthquake

By Phil Plait | March 11, 2011 10:02 am

[UPDATE: I have posted an article with more info on the earthquake and where you can donate money toward the relief efforts.]

Japan suffered a massive earthquake last night, measuring nearly magnitude 9. This is one of the largest quakes in its history, causing widespread and severe damage. Before I say anything else, I’m greatly saddened by the loss of life in Japan, and I’ll be donating to disaster relief organizations to help them get in there and do what they can to give aid to those in need.

While there isn’t much I can do to directly help the situation in Japan, I do hope I can help mitigate the panic and worry that can happen due to people blaming this earthquake on the so-called "supermoon" — a date when the Moon is especially close to the Earth at the same time it’s full. So let me be extremely clear:

Despite what a lot of people are saying, there is no way this earthquake was caused by the Moon.

The idea of the Moon affecting us on Earth isn’t total nonsense, but it cannot be behind this earthquake, and almost certainly won’t have any actual, measurable effect on us on March 19, when the full Moon is at its closest.

So, how can I be so sure?

The gravity of the situation

Here’s the deal. The Moon orbits the Earth in an ellipse, so sometimes it’s closer to us and sometimes farther away. At perigee (closest point) it can be as close as 354,000 km (220,000 miles). At apogee, it can be as far as 410,000 km (254,000 miles). Since the Moon orbits the Earth every month or so, it goes between these two extremes every two weeks. So if, say, it’s at apogee on the first of the month, it’ll be at perigee in the middle of the month, two weeks later.

The strength of gravity depends on distance, so the gravitational effects of the Moon on the Earth are strongest at perigee.

However, the Moon is nowhere near perigee right now!

The Moon was at apogee on March 6, and will be at perigee on March 19. When the earthquake in Japan hit last night, the Moon was about 400,000 km (240,000 miles) away. So not only was it not at its closest point, it was actually farther away than it usually is on average.

So again, this earthquake in Japan had nothing to do with the Moon.

Time and tide

So why would people think this is due to the Moon?
Read More

CATEGORIZED UNDER: Antiscience, Astronomy, Debunking

Does this perigee make my Moon look fat?

By Phil Plait | August 26, 2010 7:02 am


I’ve been posting a lot of extreme close-ups of the Moon, but sometimes you can learn something by taking a step back.

For example, I imagine if I went out in the street and asked people what shape the Moon’s orbit was, they’d say it was a circle (or, given recent poll results, they’d say it was Muslim). In fact, however, the Moon’s orbit is decidedly elliptical. When it’s closest to Earth — the point called perigee — it’s roughly 360,000 kilometers (223,000 miles) away*, and when it’s at its farthest point — apogee — it’s at a distance of about 405,000 km (251,000 miles).

That’s a difference of about 10% — not enough to tell by eye, but certainly enough to see in a picture… like this one, by the Greek amateur astronomer Anthony Ayiomamitis:

lunar-apogee-perigee-2010

[Click to emperigeenate.]

Amazing, isn’t it? The Moon is noticeably different! He took those images at full Moon, but seven months apart, when the Moon was at perigee (last January) and apogee (just a few days ago as I write this). It’s part of a project he does every year, and it’s pretty cool. He was able to get these images within a few moments of the exact times of apogee and perigee.

You might wonder how the Moon can be at apogee when it’s full one time, and perigee at another time it’s full. Read More

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

A marvelous night for a Moon (and Mars) dance

By Phil Plait | January 30, 2010 9:17 am

Were you out last night to see the Moon and Mars together? It was a lovely get-together! I took some pictures, and here’s the best one:

Mars is the reddish "star" to the left of the Moon. A couple of actual stars are visible as well, and the pink blob on the left is a reflection of the Moon inside the camera.

Funny, you can barely see Mars in the picture, but it was really obvious by eye. That’s because cameras see things linearly — an object twice as luminous as another will appear twice as bright in a picture — while our eyes see things logarithmically — a mathematical function that lets our eyes see a much larger range of brightness based on multiplication, not addition. It’s actually a bit more complicated than this, but the point is while to the camera the Moon was vastly brighter than Mars (about 30,000x as bright!), to my eye the difference wasn’t nearly as much (only about 10x as bright). This allows our eye to detect faint and bright objects at the same time, which a camera can’t do easily.

You may have read that the Moon looked so bright last night because it was at perigee, the point in its orbit when it’s closest to Earth. Honestly, that makes no difference to the casual observer. While it really was a bit bigger and brighter, the difference over a normal full Moon is pretty small, and you don’t have anything to compare it with. If you could have superimposed a normal full Moon next to the Moon last night you might have seen a difference, but with just the one Moon sitting there you’d never notice.

This reminds me of the time in 1999 when people said the perigee full Moon would be so bright you could drive at night without headlights! Yeah. Bad idea.

But I do hope that some of the hype got people outside and noticing the sky. It’s amazing what you can see, what lovely things await you, if you simply look up.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
MORE ABOUT: Mars, Moon, perigee
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