Mars vs. Saturn

There’s a bright-planet conjunction that you can see from about Sunday Aug. 24 onward and that’s easier than the Aug. 18 Venus-Jupiter one, because it’s in the evening sky. Mars and Saturn are 75 degrees east (leftward) from the Sun. You just need to go out three-quarters of an hour or so after sunset and start looking at the low fading sky left of where the Sun disappeared.

You may notice, farther to the left, above the southern horizon, a reddish spark. That’s Antares, the red-giant star that is the heart of the Scorpion constellation. The planet pair is in Libra, about a handspan to the right of Antares. (When the sky gets darker, and if it is free enough of light-pollution, behind Antares will appear the Milky Way – its central and widest part. And the Moon, whose glare washes out the Milky Way, is absent, being New (in front of the Sun) on Aug. 25.)

Mars is passing more than 3 degrees south of Saturn. That is quite a wide gap – three finger-widths at arm’s length. In a telescope Mars would look only about 1/600 and Saturn 1/220 of a degree wide.

Because they are wide apart, it’s not simple to say when the exact conjunction is. It’s on Monday Aug. 25 at 18 hours Universal time if you mean the moment when they’re closest together (called the “appulse”), or on Wednesday Aug. 27 at 13 UT if you mean the moment when Mars is exactly south of Saturn on the map of the sky (the “conjunction in right ascension”). So it doesn’t greatly matter which evenings you look, from Sunday onward. Mars, moving faster, will move from right to left under Saturn. Better to try on an earlier evening; as the evenings go on it will be more difficult to see them at all as they sink nearer to the setting Sun.

The two planets are at present of almost equal brightness, Mars imperceptibly brighter, neither of them at its brightest. We are leaving them behind as we curve away around our smaller orbit. So Mars is 1.35 Sun-Earth distances from us, Saturn more than 10. (A Sun-Earth distance is 93 million miles.) And yet Saturn, at its hugely greater distance, appears more than twice as wide as little Mars!

Diagram of the view some evenings later, when the Moon has come up to join the scene.  (Astronomical Calendar 2014, page 39.)
Diagram of the view some evenings later, when the Moon has come up to join the scene. (Astronomical Calendar 2014, page 39.)

Mars overtakes Saturn roughly once in two-and-a-bit years, but there’s little regularity about it, because of the great difference between Mars’s roughly two-year and Saturn’s 29-and-a-half-year orbits. Usually they pass much more closely to each other, but it happens that the next conjunction will be at almost this same date in 2016 and they will be even wider apart.

P.S. When I composed my piece about the Aug. 18 conjunction of Venus and Jupiter, I didn’t even notice that I could have benefited by looking at the bottom of page 39 of my own Astronomical Calendar 2014, where I had put a special chart and some more points. Well. When you’re trying to straighten up a description of something astronomical, you find yourself opening seven or eight computer files of different kinds – diagrams, notes, tables of numbers – in the process of understanding. Sometimes you should have taken a look at nine or ten.

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