Tag: apogee

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

Ring of fire eclipse on May 20

By Phil Plait | May 17, 2012 10:49 am

On Sunday, May 20, the Moon will pass between the Earth and the Sun, creating a solar eclipse.

However, this isn’t your usual event: because the Moon will be at apogee (the farthest point in its orbit), it won’t completely cover the face of the Sun. Instead of the Sun being totally blocked and the ethereal glow of its corona visible, we’ll see an annular eclipse, also called a "Ring of Fire" eclipse. The picture here — from the October 2005 annular eclipse — makes it clear why!

The eclipse begins at 20:56 UTC (16:56 Eastern US time) on May 20, and ends at 02:49 UTC May 21 (22:49 on May 20 Eastern time). Folks on the east coast of the US will not see the entire eclipse (for those on the extreme east coast, the Sun sets before the eclipse starts for that location [UPDATE: here's a good map to show you if you can see it or not, from the AstroGuyz site]), whereas people on the west coast will barely see the whole thing. For me, in Boulder, Colorado, the Sun will set during the eclipse, which I actually think is pretty cool. That means it’ll sink into the Rocky Mountains with the Moon still partially blocking it, which should make for extraordinary photos!

If you want to see the whole eclipse, the farther west you are the better. The western US and Japan have the longest view, as well as seeing the Sun blocked as much as possible; at the peak, about 94% of the Sun will be blocked by the Moon. Mind you, most people will see this simply as a partial solar eclipse, with the Moon crossing the Sun across a chord. But if you’re in a specific narrow path the Moon cuts directly across the Sun, and you may see the Ring of Fire. Check this interactive Google map to see that path. If you are outside the blue lines on that map, you’ll see a partial eclipse, but in between them you’ll see the annular effect. Cities like Albuquerque and Gallup in New Mexico, Reno in Nevada, and Redding in California may have the best American views.

There are many good sites with details. The NASA eclipse site as usual is the first place you should go, with tons of details. Wikipedia has an excellent article with some good graphics and maps as well.

NOTE: There are lots of great, safe ways to view the eclipse. San Francisco’s Exploratorium has a great list. Search Google for "safe eclipse viewing" for more. NEVER LOOK AT THE SUN THROUGH BINOCULARS OR A TELESCOPE unless you really know what you’re doing. Seriously. Even looking at it with your eyes can be dangerous; just wearing sunglasses can actually make it worse. So go to those links to see the best way to do this.

And if you’re looking for a place to watch the eclipse in the states, I might suggest trying a national park. The National Park Service has a list of places with great views!

I’m hoping to take some pictures myself and collect photos taken by others as well. Stay tuned!

Image credit: Sancho Panza on Flickr; Google.


Related Posts:

- INSANELY awesome solar eclipse picture
- When the Earth photobombs the Sun
- Moon bites multicolor Sun… from space!
- Last week’s solar eclipse tripled by Hinode
- ANOTHER insanely awesome shot of the solar eclipse?!
- The July eclipse, from 12,000 meters up

CATEGORIZED UNDER: Astronomy, Cool stuff, Top Post

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

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
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