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.

A little while back, I wrote about Jupiter appearing in an image from NASA’s SOHO Sun-observing satellite. I promised that it would soon appear in a SOHO camera that had higher magnification, and we’d be able to see its moons.

I am not one to break promises:

Awesome. It helps to set the resolution to 720p to see the moons when they’re pointed out.

And just you wait: in early June, Venus will appear in the LASCO C3 and C2 cameras, on its way for a date transiting the Sun for the last time in over a century. I’ll have more about that event in a few days… I promise!

Tip o’ the occulting bar to SungrazerComets on Twitter.

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.

On May 11, the phenomenal astrophotographer Thierry Legault took another amazing picture of the Sun (See Related Posts below for more of Thierry’s work that’s been featured here at the BA blog). Setting up his equipment in the south of France, he captured this truly magnificent shot of our nearest star… and when you finish picking your jaw off the floor, stick around, because your amazement isn’t done yet:

[Click to hugely ensolarnate.]

I know, right? That HUGE sunspot cluster is Active Region 1476, which has been blorting out some small flares, but nothing major. That’s a bit surprising, given how big and active the magnetic field is in those spots. Still, the cluster has grown to something like 200,000 km (120,000 miles) stem to stern, and that one big spot is 100,000 or so km (60,000 miles) across. Mind you, the Earth is about 13,000 km (8000 miles) across, so keep that in mind when you’re looking at it.

But there’s more to see! Including the reason Thierry took this picture in the first place…

(more…)

Alan Friedman is a photographer who takes amazing pictures of the Sun. While others were out celebrating Cinco de Mayo this past weekend, he was outside taking another jaw-dropping image of the nearest star in the Universe:

Yegads! Click to ensolarnate, and he has a greyscale version, too.

I love the detail and texture of his images. He has an excellent telescopic setup which yields the superb resolution, and he employs an old trick to get the texture: he inverts the image of the Sun’s disk, making black stuff look white and vice-versa. This is a technique that’s been used by astronomers for decades to enhance images; our eyes see details better that way. When Alan does it, I swear it makes the Sun look like a 1.4 million-kilometer-wide shag rug.

All the way on the left, just on the Sun’s edge, you can see a group of sunspots just rotating into view. That’s Active Region 1476, and Alan provided me with a clear picture of them (no tom-foolery) which I’ve put here. That monster group is about 100,000 kilometers (60,000 miles) across, so when I saw them I immediately suspected trouble.

… and sure enough, they had a medium-sized eruption just this morning. At 13:00 UTC they blasted off an M1.4 class flare; big enough to potentially cause some radio disruption and maybe some aurorae. NASA’s Solar Dynamics Observatory got a dramatic view of the eruption:

Flares this size are relatively common; there was one in late March for example. Bigger ones happen less frequently, though again we did see one 50 times this powerful in March as well! We’ll have to see if today’s eruption will cause any aurorae, and either way, we should keep our eyes on AR1476.

Image credit: Alan Friedman, used by permission. Tip o’ the Sun visor to Camilla Corona SDO on Google+ for the video.

The Sun is feisty. Rising and falling packets of ionized gas (called plasma) below its surface generate fierce magnetic fields, which store vast amounts of energy. This can give rise to such features as sunspots, explosions like flares and coronal mass ejections, and huge, towering plumes of plasma called prominences.

While observing the Sun yesterday, April 29, my pal and friend of the BA Blog Alan Friedman captured an amazing sequence of shots of an eruptive prominence, one that doesn’t simply fall back down to the solar surface, but also blasts material out into space:

[Click to greatly enfilamentate.]

Wow! Alan estimates that at its peak the eruption was 150,000+ kilometers (100,000 miles) in height — compare that to the size of the Earth, a mere 13,000 km (8000 miles) in diameter. Yowza.

He also made a color image of it which is lovely and terrifying…. and slightly familiar. It didn’t take me long to recognize it. That treacherous profile, that conniving nose, that sinister haircut…

Oh, it’s clear who’s really behind this eruption:

Exxxxcellent.

[UPDATE: Good news, everyone! When I posted this on Google+, commenter Artemis Entreri mentioned it looks more like Professor Farnsworth. I can't believe I didn't think of that myself! I blame Wernstrom.]

Image credit: Alan Friedman, used by permission; The Simpsons™ & © Twentieth Century Fox Film Corp. All Rights Reserved. Used under The Fair Use Act. And yes, I’ll admit it looks more like Abe Simpson than Burns, but if I used Abe I could’t write the headline I did. And the nose is definitely Burns’. Also? BURNS. Because it’s hot. So clearly this was the correct choice.