Astrophotographer Guillaume Poulin sent me this wonderful and lovely picture he took of the Venus/Jupiter conjunction on March 11, 2012:

[Click to embiggen.]

I love the colors of the sky in this! I also like the theme of pairs; the planets, the humans, and the barns. This was taken near the Mont-Mégantic Observatory in Canada, right after sunset, so the silhouettes of Guillaume’s girlfriend and her brother are nice and sharp. Simply beautiful.

And if Guillaume’s name is familiar, that’s because he and Rémi Boucher created an amazing time lapse video I featured here not long ago of the eastern US coast as seen by the space station. I sure hope he keeps making such beautiful imagery so I can feature it here.

… and the two planets are still making a spectacle of themselves to the west. Make sure you go look!

If you’ve been outside just after sunset and faced west, you’ve almost certainly seen two bright "stars" glaring back at you. Those aren’t stars: they’re Venus and Jupiter, the 3rd and 4th brightest natural objects in the sky (after the Sun and Moon, of course, and I say "natural" because the space station can in fact look brighter than both). As they circle the Sun, every year or so the orbital geometry of the two (and the Earth) sets things up such that they appear very close together in the sky. This is called a conjunction, and when it involves planets this bright, it’s really something to see!

[Click to enaphroditenate.]

That stunning shot was taken by German astrophotographer Robert Blasius, and it’s part of an incredible set of photos gathered by Astronomers Without Borders.

Getting pictures of the two planets together is not hard; I rested my phone on a chair at a party in the middle of downtown Austin, Texas, and got a decent shot myself, shown here (click to embiggen). If you have a real camera, great pictures are a tripod and a click away.

The two will be close together for the next few days, too, so you have time.

I’ve been getting a lot of email and tweets about the pair, asking me about them. Venus is the brighter of the two planets, which is interesting: it’s far smaller than Jupiter, and reflects less sunlight overall. So why is it so much brighter?

Because it’s closer!

The brightness of a planet in our sky depends on a handful of things: how big it is, how reflective it is (an icy object is brighter than a rocky one), how far from the Sun it is, how far from us it is, and even our viewing angle. These can all be calculated, so what the heck. Let’s do a little math!

Area of my expertise

The total light reflected by a planet depends on the surface area of that planet, which depends on the radius squared (remember your algebra? The area of a circle is π x (radius)²). Jupiter has a radius about 71,500 km, and the radius of Venus is about 6050 km.

Squaring those two and getting the ratio, we get

area of Venus / area of Jupiter = 0.0072

So all things being equal, Venus should only be 0.0072 times as bright as Jupiter, or, flipping it, Jupiter should be about 140 times brighter than Venus! But all things are not equal…

A planet’s day in the Sun

Jupiter is farther from the Sun than Venus, and that means it gets less sunlight than Venus does. The amount of light an objects gets from the Sun depends on its distance from the Sun squared (this is called the inverse square law of light). We can figure that out too! Just divide the distance of Jupiter from the Sun by Venus’s distance from the Sun, and square that:

Jupiter’s current distance from the Sun is about 746 million km, and Venus is about 108 million km. So,

(746 million / 108 million)² = 47.7

So Venus receives about 47.7 times as much sunlight as Jupiter does, and all things being equal should be 47.7 times as bright. But we’re not done…

Long way home

The light from each planet also has to get from there to here, to Earth. Again, that goes as the square of the distance! So we have to account for that. Right now, Jupiter is about 846 million km from Earth, and Venus is about 121 million km distant. Let’s square those and see what happens:

(846 million / 121 million)² = 48.9

Yikes, another factor of almost 50! So again, due to their respective distances from the Earth, Venus should be 48.9 times brighter than Jupiter. But wait! There’s more …

Pause to reflect

Both Venus and Jupiter are highly reflective — both are covered in clouds, though the clouds of Venus are somewhat more reflective than those of Jupiter. Venus has a reflectivity (called its albedo) of about 67%, and Jupiter is about 52%. That’s pretty close. The ratio is

.67 / 0.5 = 1.29

So Venus wins there too.

And still there is one more thing: phase! We see Venus roughly "half full" right now because it’s off to the side of the Sun, while Jupiter is "full". According to the US Naval Observatory, 57.6% of Venus is lit right now from our view. So we have to account for that by throwing a factor of 0.58 on Venus.

And now we’re ready to put them all together!

So let’s figure out how much brighter Venus is than Jupiter by putting these all together. All we have to do is multiply them all, making sure we do it in terms of Venus to be consistent:

Venus brightness / Jupiter brightness = 0.0072 x 47.7 x 48.9 x 1.29 x 0.58 = 12.6

So Venus should be a bit less than 13 times brighter than Jupiter according to my calculations.

Is that right? Looking up the actual numbers on Heavens Above, I get that Venus is actually about 8 times brighter than Jupiter. My number’s off, but not by much. That’s not bad, considering I just threw some numbers together based on straight geometry and did some very rough math.

I love this aspect of astronomy: using nothing more than a few first principles — how light gets dimmer with distance, how objects reflect that light, and how we see them — I was able to predict that Venus should be brighter than Jupiter, and even by how much! The actual amount may be off, but I wasn’t wrong by a factor of ten, say. Just that basic knowledge got me pretty close, and no doubt there are a handful of other factors that, when accounted for, get this much closer if not bang on the right number.

After all, the Universe knows what it’s doing. And science is a great way — the best way — to understand what the Universe is doing. Make a few assumptions, test them out, see how well they fit what’s really going on, and then look more deeply into the problem. I’ll hold off here from going deeper into the numbers above (though I’ll bet I see some folks in the comments diving in), but this is a great example on how science zeroes in on a solution.

And honestly, you don’t have to do all the math to go outside and take a look. Please do! The math isn’t hard, but what’s easier is facing west and looking up. That is the heart of astronomy.

Look up!

Image credit: Robert Blasius/Astronomers Without Borders; me

The Moon and Venus make a pretty pair, don’t they?

I took this shot myself an hour ago as I write this, about 17:00 local time here in Boulder. I used my cell phone camera, then in Photoshop cropped it to 610 pixels wide (the biggest my blog width will allow) and blurred it a tiny bit to reduce the background noise. You can just barely see the "dark" part of the Moon, lit by reflected Earth light.

The pair were closer together last night, and the Moon’s motion around the Earth are separating them more as you read this. But they’ll be back together again on January 26th (they’ll get about 6° apart, 12 times the width of the Moon), and even closer on February 25th, when they’ll be about 3° apart! That’s a really nice photo op, so be prepared for it. If I can get such a nice shot with just my crappy phone, imagine what real photographers with nice equipment can get. I hope to see lots of gorgeous pictures of the pair then.

Just an hour or so ago as I write this (Saturday, November 26, 2011) I was sitting at my desk at home, puttering around on the computer. I glanced out my office window and noticed the Sun had set a few minutes before. Even though it was still quite bright out, I thought I might be able to spot Venus low in the west. So I leaned back and looked out the window. Venus was easy enough to spot — it’s really quite bright — but to my surprise and delight a very thin crescent Moon was hanging right next to it!

I did two things right away: I tweeted about it, so others could go outside and see the pair if they could, and then I grabbed my camera and went outside. I took literally 111 pictures, and put the best of them on Flickr. Check this one out!

[Click to embiggen.]

This was one of the first of the set I took; the sky was still quite bright. You can see the very young Moon on the right, and Venus way over on the left. I measured the distance off the picture, and they were about 3° apart, or about 6 times the width of the Moon’s face. That’s pretty close!

I kept snapping away as the sky darkened, and moved around a bit to get a more interesting foreground. I like the way this one came out:
(more…)

We live on a spinning ball, rotating madly as it moves through space. Once every day the surface of our planet makes a circuit around the imaginary line connecting its poles… well, imaginary it may be, but the effects are quite real, especially when you take long exposures of the night sky. That’s what photographer Brad Goldpaint did, and created this lovely time lapse video he calls Breaking Point:

[If you go to the Vimeo page for the video you can watch it in HD, which you really need to do, as well as make it full screen.]

Amazing, isn’t it? The visual of the stars wheeling around the sky over our head invokes such a wonderful feeling, as if the whole Universe is spinning around us. But it can also be a little odd-looking too. For example, take a look at this picture Brad composed using some of the images he crafted into the video, which he has singled out and called Delineated:

[Click to siderealate.]

Strange, isn’t it? For one thing, it isn’t one long exposure, but instead composed of 60 short exposures added together. If you squint you might see streaks of light, but in reality those arcs are composed of individual dots, the images of stars frozen as they moved across the sky.

It’s also a bit odd due to the fuzzy glow at the bottom. That’s actually the smeared-out light from the Milky Way galaxy as it rose into the frame. Not being a point-like source of light like stars, it has a dreamier, fuzzier quality. Again, from the video, here’s a single exposure from that series:

(more…)

Last year, astronomers discovered a remarkable planet orbiting another star: it has a mass and radius that puts it in the "super-Earth" category — meaning it’s more like the Earth than a giant Jupiter-like planet. Today, it has been announced that astronomers have been able to analyze the atmosphere of the planet (the very first time this has ever been accomplished for a super-Earth), and what they found is astonishing: the air of the planet is either shrouded in thick haze, or it’s loaded with water vapor… in other words, steam!

[Click to embiggen the artist illustrations of the planet and star.]

This is very cool news. Um, hot. Whatever.

Here’s the deal: GJ 1214 is a dinky red dwarf star 42 light years away. It’s only about 1/5th the size of the Sun, and shines with only 1/300th of the Sun’s brightness. A project called MEarth studies such nearby red dwarfs, looking for dips in their starlight that indicate the presence of a planet: when the planet passes in front of the star (called a transit), it blocks the light a little bit.
(more…)

If you look west after sunset, you’ll probably spot the fourth brightest object in the sky^*: Venus.

But as I looked west recently, I noticed two bright(ish) objects just above it. It didn’t take me long to figure out that they were the planets Saturn and Mars. Both looked red due to their low altitude above the horizon, and both were about the same brightness, so I wasn’t sure which was which. Happily, Sky and Telescope has a map (shown here) and a write-up of what’s what.

Interestingly, although Saturn is far larger than Mars, it’s much farther away, so they appear to be about the same brightness in the sky. All three of these planets will change their positions noticeably over the next few weeks, so you can watch as the dance of gravity morphs their configuration. Also, on August 12 and 13, the crescent Moon will slide past the trio, which should make for a very nice photo opportunity.

Not only that, but if you stay up late, you can catch the Perseid meteor shower as well. I’ll have more about that later. But until then, even people who go to bed early can spot and appreciate the view to the west.