A trio of lights

Perhaps you took a look at the after-sunset sky and saw Mars passing Saturn, as we described a few days ago. If you were gazing on Monday, August 25, the Moon at that same time was just off-stage. It was down under the horizon to the right along with the Sun.

Well, the Moon then started climbing on-stage. The next evening, it was almost certainly too difficult to see, but, the evening after that, some sharp-eyed observers may have glimpsed this extremely slender “young” Moon just up in the bright sky to the left of where the Sun had set.

By the evening of Friday Aug. 29, when I’m writing, the Moon has grown to a classic crescent in shape, though still not far above the sunset horizon. And below it is a coolly white glimmer: Spica, the chief star of the constellation Virgo, and ninth brightest of all the stars of the night.

There is now a narrowing gap between the Moon and the two planets, Saturn and Mars, which had their conjunction on August 25. They are slowly separating from each other as Mars moves leftward. The Moon will sweep between them during the night between Sunday August 31 and Monday September 1.

It will pass so close to Saturn that indeed it gets in front of the planet – occults it – as seen from eastern America and the Atlantic. This happens at 19 UT (Universal Time, Greenwich time), which for America unfortunately is in the afternoon at 15 EDT (Eastern Daylight-Shifting Time). About six hours later, thus in darkness for America, the Moon is closest (about 4 degrees) north of Mars.

The three bodies, Moon, Saturn, and Mars, get into their closest grouping at 21 UT (17 EDT). At that time they fit within a circle 4.9 degrees wide. We call these occasions, when three of the major lights of heaven fit within a 5-degree circle, “trios”. This term (owed to Jean Meeus of Belgium) is a useful way of drawing attention to some of the most interesting of the patterns that form in the sky.

This is one of four trios in 2014, though two of them are unobservably close to the rising Sun (and one involved dim Pluto). The other good one was when the Moon, Mars, and the star Spica huddled together, in an even smaller circle, on July 6.


The diagram shows the grouping on Aug. 31 at 45 minutes after sunset for eastern North America (which is just in September 1 by Universal Time). The dashed circle is 5 degrees wide. The Moon is shown in two places; why? The small dashed circle is where it would appear if seen from the center of the Earth; the white image is where it appears as shifted by parallax from this location on the Earth. Parallax makes a big difference to the position of the Moon, because it’s so close to us. The dots for the two planets are vastly larger than their real size in a telescope.



4 thoughts on “A trio of lights”

  1. I forwarded Guy’s Blog on the Trio of Lights to my Astronomy Class on Saturday to remind them of our optional meeting at Wrightsville Beach on Sunday night to view this event and to learn about the Night Sky. Only 2 students showed up which I consider not bad since it was Labor Day Weekend and the parking even after dark at the beach was almost impossible. What a shame that it takes so much effort to have a dark viewing site.

  2. The big question for viewers in Virginia is the weather, will it cooperate? The crescent moon sliding near Saturn and Mars will give a great scene, especially through binoculars!

  3. Ah, yes,,, I tried to get out Wed, to catch an early sliver of new moon, but it was cloudy towards the west. Thursday proved MUCH better and was a little surprised to see Mars and Saturn a little higher than I expected. Missed whatever happened last nite, but look forward to a nice tight grouping tonite.
    It’s fun to look at the planets and figuring out how we’re all moving, seeing Venus speeding behind the sun, therefore breaking up that beautiful morning grouping with Jupiter and, last week, the moon.
    It’s all so cool, Guy, keep it up :)

  4. I make a practice of looking for the Moon every day and night. Here in San Francisco, California we’ve had a remarkably cloudy and foggy summer, but there have been enough breaks to follow Mars and Saturn’s pas de deux past Alpha Librae. And I’ve been able to see the waxing crescent Moon both Thursday and Friday evenings, which has made me happy. (This morning, Saturday, it was clear enough to see Jupiter and Venus below the winter circle, which was a rare treat. Unfortunately their conjunction earlier this month was obscured here.)

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