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StarDate: Venus and Mercury

by Damond Benningfield
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FROM THE UNIVERSITY OF TEXAS McDONALD OBSERVATORY, AS HEARD ON KCBS RADIO WEEKDAYS @ 9:52 A.M., 7:35 P.M. & 2:52 A.M

STARDATE 07/11/14: Mercury and Venus, the Sun’s closest planets, team up in the eastern sky at dawn the next few mornings. Venus is the “morning star,” so it’s an easy target. Mercury is fainter, but it’s not far to the lower left of Venus, which will help you pick it out.

Mercury is a little more than a third of Earth’s distance from the Sun, while Venus is roughly two-thirds of the Earth-Sun distance. So it would make sense for Mercury to be the hotter of the two planets. And it is indeed quite warm — noontime temperatures at Mercury’s equator can reach 750 degrees Fahrenheit.

But since Mercury doesn’t have an atmosphere, it doesn’t retain that heat at night, so temperatures plunge to hundreds of degrees below zero. And at the planet’s poles, the temperature can remain below freezing all the time.

Venus, on the other hand, does have an atmosphere. It’s about 90 times denser than Earth’s, and it consists almost entirely of carbon dioxide. The combination turns Venus into a planet-sized oven, with steady temperatures of about 850 degrees — day and night, across the entire planet. That makes Venus the hottest planet in the solar system.

Again, look for Venus low in the eastern sky at dawn, with Mercury to its lower left. Mercury is easier to spot from the southern states, where it rises a little straighter into the sky. The planet will get a bit brighter over the next few days before it begins to fade and drop away into the morning twilight.

Script by Damond Benningfield Copyright ©2014 The University of Texas McDonald Observatory

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