Jun 26 2005
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Distance Learning
Note: I wrote this on Sunday afternoon, fully expecting the normal California weather to allow me to go outside and get some photographs of the planets in the western sky. But the best-laid schemes o’ mice an’ men gang aft agley, as they say (”they” being Robert Burns, in this case). It’s cloudy out. So pretend there is a picture here of three planets really close together. Thanks.
In affairs of the heart, it is said, distance adds perspective. “Give yourself time to distance yourself from the situation,” people will say, “and things will seem clearer.”
But in astronomy, the exact opposite is the case. Distance does not add perspective. It robs us of it.
Consider a nearby object, such as your thumb held at arm’s length, and a distant one, such as a tree across the street. If you hold one eye closed and look at the tree past your thumb, you may note (say) that your thumb appears to be on the left of the tree. Now switch eyes. Suddenly, you may see your thumb jump to the other side of the tree. That’s because the angle made from your eye to your thumb and again to the tree has changed. This effect is called parallax. If you want more details on this, I have page describing it more thoroughly here.
But when an object is too far away, this method fails us. The distance to the object is much larger than the separation of our eyes, and parallax using just our eyes is useless (and so we must employ other methods in that case). Such is the situation with astronomy. Astronomical objects are so far away that it’s impossible for us to directly perceive their distance. We literally lack perspective.
A case in point is hovering in the sky above me even as I write this. Three planets are making a relatively rare neighborly pass– Venus, Mercury, and Saturn are all in a part of the sky small enough to cover with your thumb, if you still have it hanging on the end of your outstretched arm.
If you’re reading this on Monday, then look to the west just after sunset. Venus is the brightest of the three, and the most obvious. Just next to it is Mercury, and below the pair and to the right is Saturn (if you are in the southern hemisphere, then reverse left and right). Saturn is faintest, and you may not see it until it starts to get darker outside (Sky and Telescope magazine has an animation of this event online which should help you visualize the event) .
Since we don’t know their distances just by looking, we might assume Venus is the closest, since it’s brightest (it’s about 33 times brighter than Mercury right now). Mercury and Saturn are roughly the same brightness, so maybe, you’d think, they are equally distant from us.
BZZZZT! Nope. Venus is actually about 1.5 times farther away from us than Mercury, even though (overly) simple math based on its brightness would imply that Venus is 1/6 the distance of Mercury. And Saturn is actually a whopping 10 times farther away!
The reason Venus is brighter is because it’s much bigger than Mercury, and reflects light better. Saturn, of course, is way bigger than Mercury (25 times the diameter, meaning over 600 times the surface area!), so even though it’s really far away, it rivals Mercury for brightness.
So, without knowing the distances to these objects we’d have no clue about their real properties. Appearances can be deceiving.
Distance may be just the solution for some emotional turmoil, but for a more cosmic perspective it’s absolutely crucial.
There’s a similar view here, only as a subset of one of the planets in this weekends trifecta:
http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1561
I’ll try my best at seeing the show tonight, but that’ll be a tough one. Finding Mercury through the trees low here was something earlier on. But I’ll probably find a nice spot without too much trouble this time!
Mercury is so close to Venus that if you find Venus, you’re in.
The weather can be very unkind. If you pop over to my blog, you can see some images of the massing that I managed to snatch inbetween rain storms. Astronomy blog has a nice image too.
In regard to the parallax phenomenon, don’t astronomers use that technique to measure or calculate distances to distant objects by observing them from Earth at six month intervals? That’s a cool way of separating our “eyes” by the width of our planet’s orbit!
Thanks for all your time and efforts, Phil. I enjoy your work very much on Bad Astronomy as well as Night Sky mag.
I’m confused. You said that an observer in the southern hemisphere should reverse left and right. For objects in the western sky, would they really be reversed? (I understand how something in the southern sky in Ohio would be in the northern sky in Chile, thus switching left and right, since the observers would be facing each other. I’m thinking that this is different though.)
saw it…awesome; Took my 7 year old (who has a reasonable interest in astronomy) to show it with a 8 inch Dob. He was mesmerized.
Mark Bukal, plane of the ecliptic? The planets are roughly in line. While the Earth’s axis is tilted, there’s a definite plane that matches the plane of the ecliptic that cuts through the Earth. That plane will define a North and South. North of that line, the planets will be south of you and fall more left as they are higher in the sky. South of that plane the planets will be north of you and fall more right as they are higher in the sky. The same way the Sun’s path in the sky is either slightly south or slightly north for most people, and not directly overhead.
Here’s a nice picture of the planets from from Mount Hamilton:
http://antwrp.gsfc.nasa.gov/apod/ap050630.html
As to the directions being reversed in the southern hemisphere:
Being in the southern hemisphere could be compared to standing on your head. If you stand on your head facing the same direction as when you were standing, left and right will switch directions from your previous viewpoint.
Although now that I think of it, I suppose it would be more precise to say that the their relative positions with respect to the horizon would rotate the same number of degrees as the difference in latitude. You’d have to go from the north pole to the south pole to completely reverse left and right.
Venus may also be brighter due to the planet’s phases. Since Venus is farther away it should be on the far side of the sun and therefore has most of its sunlit surface facing us. I don’t know what phase Mercury is in though. The lit side could be facing us or away from us.
I couldn’t get a good dark sky / clear horizon / cloudless sunset. But then, I think the UK is one of the hardest places to find a dark sky.
The faintest object I’ve ever seen from the UK was the Ring Nebula when it was nearly overhead … until it was occluded by a power cable (I then lost the track).
Still, the linked photo (from Bruce Langdon’s comment) is very nice.