Sunday, October 14, 2007

can you spot mercury?

jupiter keeps sinking lower and lower in the early evening sky, but it is still bright and clearly visible in the southwest for a few hours after sunset. tonight, the new moon is three days past new and is just visible above the horizon at dusk as it sets just after the sun. if you have a clear view of the horizon, look for the waxing crescent moon and see if you can spot mercury to its lower right.

you'll need a pair of binoculars and a very clear view of the horizon, but its worth looking, because mercury is rarely visible to us on earth! mercury is the closest planet to the sun so from our perspective on earth, the little planet always remains very close to our bright star as it follows its orbit around the sun. in fact, in our sky, mercury never appears farther than 28 degrees from the sun, so it's only visible just as the sun sets or rises. the angle shown in the image below is about 90 degrees, but it gives you an idea of what i mean. mercury never moves in our sky farther from the sun than about a 3rd of the angle shown below.

in the image below (generated at solar system live), you can see the sun, mercury's orbit next, then venus, earth and mars. these 4 inner planets are also called "terrestrial planets" because they are solid and rocky. you can see that as earth orbits around its orbit, it always sees mercury close to the sun.

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