Moon-Venus-Mars triangle

The Moon passes closest to Venus

on January 31 at 17 hours Universal Time, and closest to Mars about 10 hours later (Feb. 1, 2:30 UT).

So the circle embracing the three of them shrinks to its smallest diameter (5.45°) at 22 UT, which is 10 PM in Europe.  For America that moment falls in daylight (4 PM EST, 2 PM PST) but after dark the Moon-centered triangle is scarcely different.

The Moon at this time is about 59 Earth-radii away (376,000 kilometers), Venus 0.54 astronomical units or Earth-Sun distances (80,781,000 km), and Mars 1.85 a.u. (276,756,000 km).

As for their brightness, by the way we measure it in astronomy the “magnitudes” of Moon, Venus, and Mars are -8, -4.6, and +1.1.  So you might think that Venus is a good deal above the middle – nearer in brightness to the Moon than it is to Mars.  I’m not sure it really looks that way; this may be a subjective matter.  Each step of magnitude means about 2.5 times more light, and it may be that the upper steps seem to increase a body’s brightness more noticeably.  The steps between stars of magnitude 4 and 5 don’t seem as great as those between planets of -1 and -2.  Or what do you think?

In our picture of the scene, the Moon is exaggerated 4 times in size, and Venus 150 times so as to show the crescent shape your telescope would find it to have.

(Look closely and you should see a dot: it’s at only 15 – instead of 150 – times Venus’s true size!)

Moon and Venus have similar crescent shapes (of vastly different sizes) because they have arrived at similar angles in relation to us and the Sun.  But they have arrived that way by moving in opposing directions: Venus is curving toward us, around and in, to pass between us and the Sun: the Moon is curving out around us from its New toward its First Quarter direction.  So from their present positions the Moon will depart rapidly leftward, and Venus rightward, not so rapidly, though rapidly compared with the motions of the more distant planets.

You could take another look at Venus’s motion in the spatial picture with yesterday’s post.



4 thoughts on “Moon-Venus-Mars triangle”

  1. Jan. 31–Thank you! I used your illustration of the moon-Venus-Mars triangle to orient myself this evening when the space station was due to pass over. Here is the report I emailed to my children. It was captioned “Like a tea-tray in the sky”.

    “Slithering around at the bottom of the driveway in the snow at 6:37 p.m. this evening, looking up at the moon-Venus-Mars grouping to orient myself (if you can use that expression when looking southwest), I saw, creeping up from low in the west, a bright light where no celestial body ordinarily is; as NASA’s spot-the-station site predicted, it appeared 28 degrees above the horizon and started to climb to a maximum of 81 degrees. I grabbed onto the recycling bin (Tuesday is fortunately the night we put the garbage out) so as not to fall over, and followed the light as far as I could without my head falling off, then turned around and picked it up again as it reached its greatest height and watched it sail off and disappear in the northeast. It was surprisingly large and bright–bigger and in lower orbit than whatever we had last observed, I guess, Or maybe my eyes were watering from the cold.”

  2. Guy, Thanks for your dedicated work. As always it is extraordinarily well done and I appreciate it. Sorry to say, the weather prediction for the Miami Valley of Ohio’s continued cloudy.

  3. A celestial object’s apparent size seems inversely related to it’s subjective brightness. In this case, Venus, appearing as a point of light (or nearly so) to the unaided eye, looks much more brilliant than the Moon, whose light is spread out over a relatively large area in the sky, and therefore on my retina.

    This past Saturday night my astronomy club had our monthly star party on Mount Tamalpais. The weather was perfect: clear, dry, and calm! After the sky became fully dark, Venus was so bright as to be somewhat distracting, even a bit annoying. Venus, goddess of beauty, doesn’t like to be ignored. But when she finally set after 10 pm, we all sighed in relief.

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