November 24, 2017

Astro-Event: Mars at Opposition.

Contrary to the once-every-August viral emails soon to be clogging your inbox, Mars will not appear as “large as a Full Moon” on this or any other year… but Mars will reach opposition this week on Friday, January 29th. Unfortunately, this apparition isn’t a particularly favorable one; Mars will only reach 14.1” arc seconds in apparent diameter, a far cry from the excellent 2003 opposition where it reached 25.1”, very close to the possible max. This is due to the fact that while Earth reached perihelion earlier this month, Mars is also very close to aphelion in its relatively eccentric orbit. In fact, although Mars approaches us every two years or so, the next really good opposition won’t be until 2018 (24.3”).  Still, any opposition of the Red Planet is worth viewing, as it is rare that Mars reveals any detail at all! Mars is currently in the constellation Cancer, and rises low in the east after sunset. Shining at magnitude -1.2, Mars is unmistakable for its orange-to red glow. Do things look a bit yellowish? A planet wide dust storm could be underway, as it is entering spring on the northern hemisphere of Mars.

A minor plus for northern hemisphere viewers is Mars’ northern declination of +22° degrees; this means that Mars will be well placed for viewing and imaging virtually all night. To view surface detail, you’ll require optics of 3” or greater; the great northern polar cap is probably the first detail you’ll see. It appears as a whitish splash on the limb of the planet. To see more, you’ll need larger optics, high magnification, clear, steady skies, and patience. Don’t forget that the Martian day is only 40 minutes longer than ours; this means that if you view at the same time each night, the planet will have shifted by only 10 degrees in western longitude. I’ve seen some pretty decent stop motion gifs made this way…now is the time to brush up on Martian geography. For those with monster (8” or larger) scopes, catching the Martian moons of Phobos and Deimos is not out of the question… more on this in next week’s post!

The astro-word for the week is retrograde motion. Follow Mars in its nightly motion against the background of stars over a period of weeks, and a curious thing occurs; after moving steadily westward, Mars will appear to pause and become stationary around opposition then start moving backward as seen against the sky. Then in the period of a few weeks, normal forward motion will resume. This odd behavior is an illusion; Earth is moving twice as fast as Mars on its inside track around the Sun, and Mars’s apparent backwards track is the result of us overtaking the Red Planet. All planets undergo an apparent retrograde motion, but Mars’ is the most apparent. Of course, this aberration presented a major problem to pre-Copernican geocentric astronomers, who wished that this pesky effect would just go away. Unfortunately, the universe rarely gives us neat, tidy circles!


  1. [...] why consecutive oppositions are only on average 2.13 years apart! The last opposition occurred on January 2010 in the constellation Cancer, and the next won’t occur until April 8th, 2014. Note that oppositions [...]

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