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.

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.

Because you simply cannot have enough incredibly beautiful photographs of aurorae in your life, here’s one taken near Tromso, Norway, on March 28, 2012 by photographer Helge Mortensen:

[Click to coronalmassejectenate, and you should.]

What a shot! Dead center in the picture is the Pleiades, the small cluster of bright stars. The bright object is the Moon, and to the lower right is Venus. If you look carefully, just above the horizon, lies Jupiter. To see it, start at the Pleiades, let your eyes move down and to the right to Venus, then keep going; Jupiter is in line with the clouds, just at the edge of the aurora itself.

I love how that one long swooshing ribbon of aurora cuts across the whole picture. See how it looks broader to the left, then narrower as you follow it to the right? That’s almost certainly perspective making it looks smaller. It’s probably something like 100 kilometers (60 miles) above the Earth’s surface and follows the Earth’s curve. The far end of it, near the horizon, is much farther away than the part at the upper left.

And despite all the drama occurring in the sky, my eye keeps getting drawn to the water. In this 10 second exposure, the slow movement of the water softens its appearance. Funny, too: I saw a face in the water and chuckled, then noted that Mortensen got a note from a friend who saw the face as well… or maybe a different one. But the one I see is pretty obvious. Do you see it too?

Mortensen has many more beautiful shots of aurorae on his 500px page, so head over there and soak up the glory of the active sky.

Image credit: Helge Mortensen, used by permission.

Well, now I feel bad: when I deflated the Supermoon stuff over the weekend, I swear I didn’t mean it literally!

This amazing shot was taken by astronaut André Kuipers from the International Space Station on May 5, 2012, as the perigee full Moon set behind the Earth’s limb. The Earth’s atmosphere bends light from the Moon, acting like a lens, pushing the bottom part of the Moon up into the top.

Science once again saves me from embarrassment. I was pretty sure the Moon wouldn’t take it personally.

Image credit: ESA/NASA

So, tonight is the so-called Supermoon, when the Moon happens to be full at the same time it’s at perigee, the point in its orbit closest to the Earth. This makes it somewhat larger and brighter than normal, and that’s getting a lot of attention in the press. I pointed out a few days ago that in reality, you almost certainly won’t notice the difference between this full Moon and any other, mostly because the difference is small, and our eyes and brain are terrible at judging things like that without something to directly compare it to.

I was thinking about this last night as I watched the almost-full Moon rise in the east (which, I’ll add, ironically looked huge due to the Moon Illusion!), and thought of something that might help illustrate this last point.

Monetary eclipse

Imagine you go outside tonight to look at the full Supermoon rising in the east. Imagine also you’re holding a US dime in your hand (if you live in another country, feel free to substitute your local currency, but beware of the math; hang on a minute to see).

Let me ask you this: How far away would you have to hold the dime so that it appears as big as the Moon to you?

A few inches? A foot? (Convert to metric if you wish). Go ahead, guess!

… OK, ready? [Answer is below the fold so as not to spoil it.]

(more…)

I give talks about asteroid impacts quite often, and sometimes people ask me why we should worry about them. I reply, "Go outside and look at the Moon. Then tell me we don’t need to worry about asteroid impacts!" The Moon is covered in craters, and it really brings home — literally — the fact that we need to understand impacts better.

I’m not being facetious, either. Looking at the Moon is a great way to learn about craters. By measuring their size, position, and shape, we can find out a lot about the history of impacts in the Earth-Moon system. The problem is there are so many craters — billions, if you look at high enough resolution. How on Earth — haha — can any scientist or team of scientists possibly look at them all?

Well, it depends on how big the team is. Enter citizen science: non-professional-science people who nevertheless love science. If you’re reading my blog — and you are — then that means you! CosmoQuest.org is a group of astronomers, run by my friend Dr. Pamela Gay, who have created a series of projects where people like you can perform needed tasks that are real science… in this case, measuring craters on the Moon! Using MoonMappers, you can identify and measure craters using images from the Lunar Reconnaissance Orbiter, a spacecraft currently circling our Moon and taking thousands of high-resolution pictures.

I signed up and started right in, and find it somewhat addicting. You’ll need to register first through the CosmoQuest forum, which takes one minute and is free. Once you’ve done that, just go back to Moonmappers and dive in. I was able to identify dozens of craters in just a few minutes. Here’s a typical scene:

The blue circles are craters found using automated software. The green ones mark craters I found. The task is really simple: you can mark craters with your mouse, dragging the circle to match its size. If you miss a bit, you can easily adjust the circle’s position to re-center it. You only need to find craters bigger than 18 pixels in size, so it’s not an impossible chore! You can also flag odd features like linear cliffs, boulders, and so on, if you happen to see any. Several of the images I went through had them. One had lovely striations in an old lava flow, so you never know what you’ll see.

Sound like fun? It is! But hurry: right now, CosmoQuest has issued a Million Crater Challenge, to get 1,000,000 craters identified by full Moon, which is on May 5, just days away. As I write this they’re still a long way from their goal. How many can you find?

And remember: this isn’t just fooling around, this is real science. How are craters made? Why are they different shapes? How many are 10 meters across versus 20 versus 30 versus 100? All these questions are important in understanding impacts… especially that last one. Getting the scales of impacts, and how the numbers of them increase as the size gets smaller, is critical in being able to predict how often they happen. At some point, we’ll see a small asteroid headed toward Earth, and we’ll have to decide if it’s big enough to worry about and spend hundreds of millions of dollars deflecting it. The work you do here, quite seriously, can help inform that decision.