The Backyard Astronomer – December 2019 – The Conjunction of Mars and the Moon

    Our planetary neighbor Mars will be making a close approach to the Moon in the wee hours of 23 December.  This is a great time to spot planets as the moon is the biggest and brightest object in the night sky – and subsequently the easiest for the backyard astronomer to locate.  We call this cosmic rendezvous a conjunction.


    Sometimes a conjunction with the Moon can be less than desirable for amateur astronomy, as the brightness of the Moon can make it hard to view the much dimmer objects around it.  This conjunction will be a little different, as the Moon will be 27 “days old” – a term astronomers use to reference the time in the Lunar cycle. In approximately 29.5 days the Moon completes one cycle or synodic month, usually counted from one New Moon to the next.  So, when we say the Moon is 27 days old, it means that the Moon will be just a thin sliver of light and not a fully illuminated orb. Being much dimmer now, the object in conjunction is not as washed out by reflected light and easier to view.

    Rising at 4:33 AM (MST) in the Prescott Area and into the constellation Libra, the waning crescent Moon will be at magnitude -10.4, and Mars at magnitude 1.6.  By comparison, the Sun is a magnitude -27 and Sirius, the brightest visible tar in our night sky, has a magnitude of -1.46. So, while Mars will be dim, it will still be one of the brighter objects in the sky that night.  Mars will be 3°31′ to the south of the Moon.  Look for the red dot!

    While a large telescope would not fit both the Moon and Mars it its field of view, you should be able to easily locate the red planet with the naked eye.  Large binoculars or a small telescope will quickly reveal the rust colored surface, while larger telescopes will begin to resolve variations in surface features, and the largest backyard telescopes even giving hints of the Martian polar ice caps.  These polar features are similar to what we have on Earth, though composed mostly of frozen carbon dioxide, more commonly known as Dry Ice.

    Adam England is a local insurance broker and small business consultant who moonlights as an amateur astronomer, writer, and interplanetary conquest advisor. Follow his rants and exploits on Twitter @AZSalesman or at 

    5% sliver moon courtesy Jeff Stillman,

    Mars courtesy Joel Cohen, Prescott Valley 21 May 2016.

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