A “trio” forms in the evening sky – a grouping of three notable bodies within a circle of diameter less than 5 degrees.
The last good trio was back on August 31, a gathering of the Moon, Mars, and Saturn. This time the third body is remote Neptune, invisible to the naked eye, so the spectacle is partly for binoculars or the mind. And it is lower in the post-sunset sky: the elongation (angular distance from the Sun) is about half as great, 36 degrees instead of 71. Still, the evening scene is a pretty one, with brilliant Venus lower down, and Mercury perhaps discernible.
In this picture, the Moon is shown also where it will be at this time on preceding and following days. Its position is corrected for parallax (that is, if we were looking from farther south the Moon would appear farther north). The picture is drawn for longitude 0 (Greenwich); for North America the only noticeable difference is that the Moon will be about a quarter of the way on toward its next day’s position. Small arrows show how the planets and Sun are moving from day to day against the starry background.
Here’s the sequence of events that weave this pattern. Mars passed close south of Neptune on the night between Jan. 19 and 20. The Moon will pass closest north of Neptune at Jan. 22 23 Universal Time, which is 11 PM in Britain, 6 PM in eastern North America, 3 PM on the west coast. Two hours later comes the moment of the tightest grouping of the three (the circle shrinking to diameter just over 4 degrees). Another two hours later, the Moon passes closest north of Mars. So what most of us in the Atlantic world see this evening is the prelude to the trio; the trio itself tightens after it has sunk below our night horizon.
It happens that around the same time, actually on Jan. 21, the Moon is at its perigee (closest to the Earth) and Mercury is at its perihelion (closest to the Sun).
The Moon will be at its descending node (slope south across the ecliptic) on Jan. 25 and occult (pass in front of) Uranus shortly afterward, just off our picture.
The Moon this evening is 56.6 Earth-radii away (361,000 kilometers), Mars about 2.07 astronomical units (310,000,000 km), Neptune 31 AU (4,600,000,000 km).
For good measure, I’ve included periodic Comet Finlay, which will be far dimmer than Neptune, almost as dim as Pluto, and yet one of the brighter predicted comets for this year. It is already receding from the Sun, but only just past its nearest to Earth. So you might stay out as the sky darkens and try to find it telescopically, using the chart in the “Comets” section of Astronomical Calendar 2015.