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From: dipper@utastro.UUCP (Debbie Byrd)
Newsgroups: net.astro
Subject: StarDate: July 17 Mars Behind the Sun
Message-ID: <366@utastro.UUCP>
Date: Wed, 17-Jul-85 02:00:41 EDT
Article-I.D.: utastro.366
Posted: Wed Jul 17 02:00:41 1985
Date-Received: Sat, 20-Jul-85 05:38:49 EDT
Organization: U. Texas, Astronomy, Austin, TX
Lines: 34

Today the planet Mars is traveling behind the sun from Earth.  More on
Mars at conjunction -- after this.

July 17  Mars Behind the Sun

During the night on Wednesday, the planet Mars is at conjunction with
the sun.  That's an Earth-centered way of saying that Mars is now
traveling behind the sun from Earth.

Conjunction is a word used when talking about the sky in an imaginary
way -- as though Earth is in the middle of everything -- and the sky is
a sphere surrounding Earth.  Mars at conjunction with the sun means
that Mars and the sun today have the same right ascension -- that's
like celestial longitude -- on the imaginary celestial sphere
surrounding Earth.

If the sky really were the inside of a globe surrounding Earth, then
today Mars and the sun would be right next to each other -- they'd have
the same longitude on this imaginary sphere.  But the sky isn't a
sphere around Earth.  What's really happening is that today Mars is
beyond the sun from Earth in the solar system -- traveling along
directly opposite where we are.  We can't see Mars now -- and won't
until summer is over -- because the red planet is now hidden in the
sun's glare.

Mars is now moving toward the outermost part of its orbit around the
sun -- its aphelion -- in mid-October.  So though it's traveling beyond
the sun from Earth today, it's not as far from us as it will be later
this month.  Today Mars is just under 22 light-minutes away -- on July
30 it'll be farthest from us, slightly more than 22 light-minutes
away.

Script by Deborah Byrd.
(c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin