Unless you’ve just arrived from another planet, you’d know that an incredibly rare astronomical event recently passed – the transit of Venus.
A “transit” is when a planet passes in front of its star. The word “in front of” depends on where you’re standing. Technically, what happened on the 5th/6th of June is that the sun, Venus and the earth were all lined up in a straight line. Venus appeared against the face of the sun as a tiny round silhouette.
Perhaps the most spectacular transits in the solar system are transits of our moon, seen from earth.Providentially or coincidentally, the disk of the moon and the disk of the sun, seen from Earth, are about the same size. The sun is about 400 times as far away from us as the moon, and is about 400 times bigger. This means that during a “transit”, the moon blocks out the whole disk of the sun – but only just, not always, and only if you’re standing at the right place on the earth. We usually don’t call these transits, we call them eclipses.
I tried to see the recent transit of Venus, but the weather didn’t cooperate. You can see my bet Venus Transit picture at the right.
You may have heard that the next transit of Venus is in 2117, a whopping great 105 years away. If you find that disappointing, I have great news! It’s simply not true! There’s another transit of Venus this very year, in September. It starts at about 4:30am, GMT, on the 20th of September, 2012, and finishes just under 10 hours later. The bad news is, most people will have to travel a bit to get a good view of it – it’s only visible from Jupiter. Oh, and some of Jupiter’s moons, in case you can’t get a good hotel room on the big planet itself.
If you’re stuck on earth, you can still get a good view – albeit simulated – using the excellent free software package Stellarium. Download it, and explore! Seen from Jupiter, transits of Venus last longer – ours was about 6 hours, but the one in September lasts 50% longer. This is a simple matter of geometry. get yourself a pen and paper, and do this :
- Draw a circle, and label it “sun”.
- Draw a diameter, and label it “our view of the sun”.
- Draw (lightly) two points, label one “Earth” and one “Jupiter”. These should be outside the circle of the sun, and have a full-on view of the diameter. Jupiter should be further away.
- Make two triangles by joining these points to the diameter.
- Draw a circle around the sun, and label it “Orbit of Venus”
You’ll have a picture something like the one shown below :
As you can see, Jupiter’s triangle contains more of the orbit of Venus than Earth’s triangle. The diagram is not to scale – if it were, both triangles would be incredibly long and thin. When Venus is in a triangle, observers on the distant planet can see a transit. The correct picture would be three dimensional, with cones instead of triangles. The cones move around as the planets move around, and more often than not the orbit of Venus doesn’t even touch the cone. Even when it does, Venus is usually off somewhere else, doing whatever Venus does when it’s not in transit.
If you think carefully about these cones in three dimensions, you may be able to convince yourself that residents of Jupiter also get to see more transits than we do. It’s true. If you miss the transit in September, there’s another visible from Jupiter on the 20th of July, 2042.
Transits like the recent transit of Venus promote a variety of (usually mild) emotions. Some people found the transit interesting. Some felt indifferent. Some were fascinated or excited. For me, the main emotions on the 6th of June were disappointment at the cloudy weather, combined with relief (and thankfulness) that I can always catch the view on Youtube. All mild emotions. However, coming up in only a few short decades is a transit that may make people weep for hours.
Obama wants NASA to land people on Mars by 2030. There are companies that want to mine the asteroids. Others wants to set up rocket fuel factories on the moon. You can buy a ticket on a spaceship for under a million dollars.
Perhaps – could it be true this time? Perhaps we are on the verge of an explosion of human space exploration. If so, it is possible – possible – that there could be a permanent human settlement on Mars by 2084.
Imagine, then, you are there. Millions of miles from home. Your family and many of your best friends are far, far away. You watched the Christmas videos they sent you last year, wishing you’d been there. Now, Thanksgiving and Christmas are approaching, and you know you’ll miss them all again. You can’t even talk to them on the phone, because of the time it takes light to carry their voice back to you. The best you have is something like video messages, and even an instant response can take half an hour to get back to you. You live in a small complex on a hostile world, and you need a spacesuit whenever you want to go outside. Then, one day, you aim your ‘scope at the sun, and see this.
A transit, but not just a transit. This silhouette is your home, the place you grew up. Your childhood friends and memories are there. The places you used to go hiking. The street you grew up on. The restaurnt you had your first date. Everything closest to your heart. There it is, like a punctuation mark against the human race’s nearest star. A simple tar-black spot containing your life and everything you love.
You cry.
This will be the most touching, the most moving transit ever. Even for people on Earth, some will cry, humbled at the thought that the human race has finally escaped from the cradle and is reaching for the doorhandles.
This transit begins – the disk of the Earth first caresses the sun’s cheek – on the 10th of November, 2084, at about 2am UTC. By 2:15am, the black circle is fully visible, and begins a long march across the face of the sun. Four hours later, at just past 6am, a second, smaller dot appears behind the first – the Moon. At 10 o’clock in the morning, the Earth has reached the far edge of the sun’s orb. “Please, stay”, you whisper, but the Earth’s relentless trek continues, and by 10:15am our home has vanished into the brownish gleam of the Martian sky. For another four lonely hours, the moon follows in her master’s footsteps. She reaches the edge of the sun by 12:45pm, and slips away quietly within another five short minutes.
If only for the sake of having the best possible view of this transit, the human race’s first base on Mars should be build near the equator, somewhere in the Eastern hemisphere, say near Hershel or Aeolis Mensae, or on the Elysium Planitia. Those building the base will know where these are.