Back in September I posted an image taken from the Curiosity rover showing Phobos, one of the moons of Mars, crossing the face of the Sun.
That was pretty cool. But this is cooler: video of Phobos transiting the face of the Sun seen from the rover Opportunity on September 20, 2012!
Lest it be overshadowed (HAHAHAHA! Get it?) by Curiosity, remember Opportunity is still going strong after more than eight years on the surface of the Red Planet. These shots from the elder rover are really awesome; Phobos is not even close to being a sphere and you can see its potatoey lumpiness in the animation.
Phobos is about 27 km (17 miles) across its long axis, which is small for a moon. It looks big because it orbits Mars so close in; it’s only 6000 km above the surface. It was actually a bit farther away from Opportunity when these images were taken, making it look smaller than it could be.
In fact, given its size and distance, Phobos has a maximum size in the sky of about a quarter degree, or half the size of our full Moon. As seen from Earth, the Sun and the Moon are about the same size in the sky. But Mars is farther from the Sun, so the Sun looks smaller, about 1/3 of a degree. So even at best Phobos can’t completely block the Sun.
But… Phobos isn’t in a stable orbit. Tides from Mars are dropping it down closer to the planet, making it appear bigger. In a few million years it’ll drop low enough to create total eclipses as seen from the surface of Mars. They won’t last long, since the moon is zipping along pretty rapidly in its orbit. Still though, I have to admit to a bit of delight: creationists like to claim the Earth is special, and we’re the only planet that has the right conditions for total solar eclipses. That’s not even really true right now, and it certainly won’t be once Phobos dips down a bit more.
Of course, once Phobos gets too close to Mars a few million years later it’ll crash into the surface, making the sweatiest apocalyptic scenarios dreamed up by humans look like a warm summer’s breeze by comparison. Nature! It has a way of making our fevered imaginations look like pretty small potatoes.
Image credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ. Tip o’ the rocket crane to Mars Curiosity on Twitter.
OK, this is simply too cool.
The Mars Curiosity rover has already returned thousands of images taken of the Red Planet’s landscape. But on September 13, 2012, it was commanded not to look around, but to look up, at the Sun. Why? Because Mars’s tiny moon Phobos passed directly in front of the Sun, partially eclipsing it!
Sweeeeet. I blew the original image up by a factor of two for clarity.
Technically, this is called a transit – when a much smaller body passes in front of a larger one. Usually, there’s some science that can come from this; the timing of the transit gives a better orbit for the moon (since the rover’s location on the surface is precisely known), and so on. In this case, though, we study Phobos with other orbiting spacecraft, so I’d think its orbit and position are extremely well determined.
It may very well be that this shot was taken just because it’s cool. I actually kinda hope so.
It’s not the first time a Phobos transit has been seen; in fact it’s been done several times. Here’s a video of one seen by the rover Opportunity in November 2010:
Wikipedia has more info. I’ll note that as of right now, the image above is the only one I’ve seen listed on the Curiosity raw images page (at decent resolution, that is; there are lots of tiny thumbnails, and bigger, cleaner versions should show up soon). The image was taken by the MASTCAM, which has a filter on it so it can observe the Sun. It does that for various reasons, including being able to observe how much the Martian atmosphere is absorbing sunlight.
Phobos orbits Mars pretty close in, just about 6000 km (3600 miles) above the surface of Mars – compare that to the 400,000 km distance from the Earth to the Moon! Phobos is so close that it transits the Sun pretty much every day for some location on Mars, making this something of a less-than-rare event. It’ll only be a year before it happens again at Curiosity’s location.
Still. It’s an eclipse, seen from Mars, taken by a nuclear powered one-ton mobile chem lab that we put there. I think that qualifies as pretty damn cool.
Image credit: NASA/JPL-Caltech/Malin Space Science Systems. Tip o’ the heat shield to… MarsCuriosity on Twitter!
- Curiosity’s self-portrait
- Curiosity looks Sharp
- Curiosity rolls!
- Now you will feel the firepower of a fully armed and operational Mars rover
- Gallery – Curiosity’s triumphant first week on Mars
The mighty planet Saturn is circled by a fleet of moons, each as different from the other as individual people. And as weird and alien as it seems, this Saturnian system of planet and moons have some similarities to our own Earth and Moon. For example, as they orbit Saturn, the moons can be eclipsed by it when they pass behind the planet and into its shadow.
The Cassini spacecraft caught exactly this event as the icy moon Enceladus slipped into Saturn’s shadow last October… and there’s more here than might initially meet the eye:
That’s amazing [click to penumbrenate]. But it may not be obvious what you’re seeing at first! Enceladus is the moon to the upper left. It’s about 500 kilometers (330 miles) across, and its surface is almost entirely water ice. At the time this shot was taken, it was about 26,000 km (16,000 miles) from Cassini.
If the moon were just sitting out there, half of it would be lit by the Sun, and half would be in darkness. Cassini was off to the side a bit when this was taken, so we see the hemisphere that is almost entirely lit by the Sun. The day/night line – called the terminator – runs from the upper left to lower right of the moon on its left side as seen here.
But because Enceladus was partially into Saturn’s shadow, the sunlit side of the moon that would normally be painfully bright is dusky and dark. The shadow is deepest toward the bottom of the moon. The geometry of this scene depends on so many angles! The terminator line, the position of Cassini relative to the moon and Sun, the position of Saturn, and even the curve of the planet itself as its shadow envelops the moon.
And we’re not done. Photobombing the scene at the lower right is the monster moon Titan, which is 10 times bigger in diameter than Enceladus! But it was over a million kilometers away from Cassini when this picture was taken, so it actually looks smaller than its very much smaller sibling. It appears dark, even compared to the shadowed Enceladus, because Titan only reflects about 1/5th of the light that hits it, while shinier Enceladus reflects almost all the light that impinges on its icy surface.
Funny, too: the edge of Titan looks fuzzy, because it has a thick atmosphere. Enceladus has no atmosphere, but still looks a bit fuzzy as well due to Saturn’s shadow: the planet does have an atmosphere, so the shadow itself isn’t sharp.
The complexities of understanding even a seemingly simple picture are ridiculous when that picture comes from Saturn. But that’s so often true: things do appear easy at first glance, but far more complicated when you peer more deeply. The Universe, as are human affairs, is rarely so black-and-white.
Image credit: NASA/JPL-Caltech/Space Science Institute
I’ve posted a lot of stuff about Sunday’s annular eclipse (see Related Links below), and I figured I was done… but then I got a pretty remarkable picture sent to me.
During the eclipse, in northern California, two men sent a small (6 cubic meter) helium-filled balloon up to 90,000 feet (roughly 27 km). Equipped with a camera and an ingenious system that used puffs of gas to orient the payload, they took this pretty amazing shot of the eclipse:
[Click to penumbrenate.]
That’s the Earth on the left (duh), and on the upper right you can see the eclipsed Sun! They used a solar filter to cover half the camera’s view so that they could get the correct exposure for both the Earth and the much brighter Sun.
I really enjoyed reading their story on how they set this up and executed it. I especially liked how they launched, sat around to watch the eclipse itself, then set off to find the balloon once it came back down (shredded after it popped at its lofty apex).
I love stuff like this! Basic equipment, clever people, and a can-do attitude results in something remarkable. Well done!
P.S. My friend and fellow Boulder astronomer Stuart Robbins posted a series of lovely timed sequences from the eclipse that he took in Albuquerque. It’s well worth a click!
[First: CONGRATS to SpaceX for the successful launch of the Falcon 9 and deployment of the Dragon capsule! Everything looked great and things are apparently going smoothly. You can watch the whole thing here, and I'll have more about all this in a little while. Until then, back to your regularly scheduled blog post.]
Over the past couple of days, a lot of people are passing this image around, saying it’s from the eclipse Sunday, taken by an astronaut from the International Space Station:
Here’s the thing: it’s not. It’s actually a lovely piece of artwork done in 2009 by a Japanese artist who goes by the name A4size-ska on DeviantArt.
There are plenty of clues to show it’s not real, if you know where to look. For one, the real eclipse was annular, meaning a lot of the Sun was still seen around the silhouetted Moon. That’s not apparent here. Plus, the bright Earth (and Sun!) would wash out the background stars in a picture like this, so you’d not see them, and certainly not the Milky Way (the fuzzy band under the eclipse in the artwork).
The picture is certainly realistic otherwise! The artist notes he used images from the European Southern Observatory; the Earth and Milky Way are both clearly real shots.
If you’re curious about what the view really looked like from the ISS, then here you go:
So yesterday was the annular eclipse of the Sun, and I held a live impromptu video chat on Google+ about it. I was joined by Pamela Gay, Fraser Cain, Nicole Gugliucci, and Jason Major, and we had a live video feed using astronomer Scott Lewis’s telescope. It was way too much fun! I’ve embedded the video at the bottom of this post.
We asked for pictures, and my Twitter feed overfloweth with them! I’m collecting them to put into a gallery which I’ll have up soon, but until then, watch this incredible video taken by John Knoll in his front yard in northern California:
Isn’t that amazing? What happened is that all the overlapping leaves made thousands of tiny holes that sunlight could poke through.
This acts like a lens, focusing images of the Sun through every hole — it’s how a pinhole camera works. [UPDATE: Timothy in the comments below points out that some people were confused by my wording. I can see why; I had started to explain how a pinhole camera works then decided it was too distracting and instead just linked to Wikipedia. I didn't mean the pinhole is a lens, just that you get a sharp picture if you use one. I should've chosen my words more carefully.] You can read about the details of this on Wikipedia. Here’s a similar video, too.
I’ll have the gallery up soon, so stay tuned!
Finally for now, here’s the live webcast recording. I’ll embed it here, but note it took me a long time to get it set up and running. It really gets started at 17:23, and I suggest you skim around to see the cool stuff.
[UPDATE: The hangout's over. Thanks to all who watched! I'll have the YouTube video up as soon as I can!]
I know this is last minute, but I decided to do a live Hangout on Google+ to talk about the solar eclipse. I’ve embedded the video below if you want to watch. If you want to ask questions, go to the link above or send me something on Twitter. If you leave a comment here I won’t see it!
Today’s the eclipse! I’m excited, though our weather here in Boulder has been fairly touch-and-go the past few weeks. I’m hoping for clear skies so I can see it; I got my eclipse glasses in the mail yesterday, so I’m all set. Locally, CU Boulder is holding a viewing in the football stadium! That’s a pretty nifty idea. As a reminder, the eclipse begins at 20:56 UTC (13:56 Pacific US time) on May 20, and ends at 02:49 UTC May 21 (19:49 on May 20 Pacific time).
Observing the Sun during an eclipse can be tricky, since it’s very bright and can damage your eyes. Wikipedia has an excellent article about this. Something I want to make special note of: during the deepest eclipse, when the Sun is blocked the most, is ironically the most dangerous time to look at it with your unaided eye. Your pupil dilates (opens wide), letting in more light, but the parts of the Sun not blocked by the Moon are still just as intense. That makes it easier to damage your eye, so be very careful.
Of course, you shouldn’t look at the Sun with binoculars or through a telescope! That’ll destroy your eyes — literally — so seriously, it’s not recommended. The only exception is if you have the proper equipment designed specifically to view the Sun. Sky and Telescope’s site has a great rundown of how to observe the eclipse safely, including what equipment you can use.
If you want to photograph the eclipse, again Mr. Eclipse has great stuff, and this You Tube video demonstrates making a Sun filter for your camera out of a Pop Tart bag! That’s not for your eyes; it’s just for taking pictures (and while some websites say it’s OK for cameras, your mileage may vary — and DO NOT USE THIS for binoculars or telescopes because it does not block enough light to be effective).
The picture here is of the Sun from just this morning, taken by NASA’s Solar Dynamics Observatory. It’s just to give you a reference of what the unblocked Sun looks like. There are some good-sized sunspots today, so they’ll provide a pretty contract and a nice background to the eclipse. To get a current picture, go to the SDO site and you’ll see it there (click the drop-down menu under the picture and select "HMI Intensitygram" to get the visible light view).
You can watch the eclipse online, too. Sky and Telescope has some info on that, and as I understand it NASA will have some live feeds on their Sun-Earth Connection site. The Japanese space mission Hinode will be watching the eclipse, too.
Finally, if you want a number of people to be able to see this event at the same time, the best way is to project the image of the Sun onto a wall or screen. Here’s a video with a very simple and clever method that I may try myself tomorrow. All you need is foil and a makeup (or other flat) mirror:
Cool! It’s essentially a pinhole camera with a bigger hole but a longer focal length, so you achieve the same results.
I hope everyone has clear skies and good, safe viewing of this wonderful event!
Image credit: NASA/SDO
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.