The smallest and largest planets dance in the west tonight

Bad Astronomy
The entire universe in blog form
March 14 2011 7:00 AM

The smallest and largest planets dance in the west tonight

If you have good eyes and a flat horizon, then you can spot Jupiter and Mercury this week as they pass each other in the night twilight.

As the planets orbit the Sun, we see their position in the sky change. Right now, Jupiter is (apparently) getting closer to the Sun every day, setting in the west a bit earlier every night. Mercury, on the other hand, is apparently moving away from the Sun, getting higher off the horizon with each passing day. On the evenings of March 14 and 15th the two planets will pass each other, with a closest approach of roughly 2° -- about 4 times the width of the full Moon. That's actually pretty close, and they'll make a pretty pair in the west just after sunset.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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And I do mean just. When the Sun sets, the two planets will be only about 10° off the horizon, where the sky will still be bright from twilight. Even though Jupiter will be shining at a magnitude of -2 (making it the fourth brightest natural object in the sky after the Sun, Moon, and Venus), and Mercury at -1, they might be a bit tough to see depending on your local conditions. Binoculars might help, but make sure the Sun is gone before scanning for the planets!

The image above is a rendering using software of what the scene might look like for you. This shows the view just after sunset, with the pair labeled. Funny, too: when I made this, I saw just how close the Sun was to setting due west. That means spring is almost here! The Sun sets due west on the equinox, which is March 20 this year.

Anyway, if you get the chance to see the two planets, take it! Jupiter will be a morning object after this, and won't be visible unless you get up at the wee hours. Mercury moves rapidly day after day compared to other objects, and never gets very high off the horizon. Most people have never seen it, so you should grab this opportunity... after all, it's not often the solar system's most extreme planets flirt this way.


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