A spectacular gathering of bright planets will be the chief celestial attraction in the evening sky this weekend as Jupiter, Saturn and Mercury appear to crowd into tight quarters.
Low in the southwestern evening sky just 30 to 45 minutes after sundown, we'll be treated to what might be referred to as a celestial summit meeting, a triple conjunction, with the three planets fitting within a circle smaller than 5 degrees.
Brilliant Jupiter (magnitude –1.9), medium-bright Mercury (-0.9) and dimmer, yellow-white Saturn (+0.6) will be contained within a 5-degree circle from Jan. 8 to Jan. 12, appearing closest together on Sunday evening (Jan. 10).
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What will make this array particularly fascinating is how the configuration will change noticeably from one evening to the next. This effect is primarily due to the rapid motion of speedy Mercury relative to the two slow-moving wanderers, Jupiter and Saturn. The pattern will go from a stretched-out triangle on Jan. 8 to an almost equilateral triangle on Jan. 10.
However, binoculars are strongly suggested, as they will help pick up the planets against the bright twilight sky. Jupiter will be at the top of the triangle and is the brightest of the trio, with Mercury and Saturn forming the base angles. The sides of the triangle each measure roughly 2 degrees.
This stunning spectacle might also mark the last evening view of Jupiter and Saturn; while Mercury rises over the coming days, Jupiter and Saturn will be sinking into the sunset fires. While Jupiter and possibly Mercury may be evident without optical aid, Saturn probably will not. In the evenings after Sunday, Saturn will disappear into the bright twilight first, closely followed by Jupiter around mid-month.
Related: Best night sky events of January 2021 (stargazing maps)
Planetary peregrinations
It is highly unusual for three or more bright planets to appear in the same small area of the sky.
From our Earthly vantage point, we can readily observe Mercury, Venus, Mars, Jupiter and Saturn with our unaided eyes as they revolve around the sun. Each of these planets appears to move against the starry background at their own speeds and along their own tracks. Because they move at different speeds, the shape formed by all five planets at any particular time is unique to that moment.
All of these planets and the moon closely follow an imaginary line in the sky called the ecliptic. The ecliptic is also the apparent path that the sun appears to take through the sky as a result of the Earth's orbit around it: Technically, the ecliptic represents the extension of the plane of the Earth's orbit out across the sky.
But since the moon and planets move in orbits whose planes do not differ greatly from that of the Earth's orbit, these bodies only appear relatively close to the ecliptic line. Twelve of the constellations through which the ecliptic passes form the zodiac; their names, which can be readily identified on standard star charts, are familiar to millions of horoscope users who would be hard pressed to find them in the actual sky.
But does it mean anything?
Ancient humans probably took note of the fact that the planets — themselves resembling bright stars — had the freedom to wander in the heavens, while the other "fixed" stars remained rooted in their positions. This ability to move seemed to have an almost magical quality, and evidence that the planets came to be associated with the gods lies in their very names, borrowed from ancient deities.
The skywatchers of thousands of years ago must have deduced that if the movements of the planets had any significance at all, it must be to inform those who could read celestial signs of what the fates held in store. Indeed, even to this day, there are those who firmly believe that the changing positions of the sun, moon and planets can affect the destinies of individuals and nations on the Earth.
But as for any chance that this weekend's planet gathering might have any influence on our lives in any way! No astrologer can predict from planetary alignments or any other celestial configuration when a specific event, good or bad, will occur on Earth. The planets are always shifting in and out of celestial liaisons, and astronomical amnesia allows us to forget the last time we saw them assembling for a similar performance — Mercury, Jupiter and Saturn last formed a similarly tight triangle in May 2000. We also usually fail to recall that none of the influential magical thinking attributed to the previous event ever materialized.
Last dance for the moon and Venus
If you'd like to see two other celestial bodies engage in a rendezvous, then plan to wake up early on Monday morning (Jan. 11), when the two brightest objects in the night sky will engage in a final pre-sunrise dance.
Set your alarm clock to ring one hour before sunrise, and quickly head out to a site where your east-southeast sky is free of any obstructions, such as buildings or trees, since what you'll be attempting to see will be poised low above the horizon. You'll be looking for the brilliant planet Venus and, hovering four degrees to its right, a sliver of a crescent moon just two days before reaching its new phase.
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Venus became a morning fixture late last spring, and Monday's appearance will mark the seventh time the moon and Venus have formed an eye-catching configuration since then. Unfortunately, this will be the last of such lunar get-togethers for a while.
Venus has slowly been getting lower in the dawn sky for the past four months. During January, this dazzling orb doesn't even rise until morning twilight is underway, and the planet is only about 10 degrees up — the equivalent of your clenched fist held at arm's length — by the time it fades away in the brightening sky, although it remains visible in the east-southeast until just before sunrise.
But the sooner you look for Venus this month the better; by month's end it doesn't appear over the horizon until after mid-twilight, preceding the sun into the sky by only about 40 minutes. Although Venus is still more than 10 weeks from superior conjunction with the sun, the fact that it is placed well south of the celestial equator keeps it very low as seen from mid-northern latitudes.
As Venus draws closer to the sun, the planet will drop deeper into the bright morning twilight, eventually being rendered invisible after about the first week of February. It will begin to reemerge very low in the west-northwest evening sky shortly after sunset during the second half of April.
From then on, Venus will spend the rest of 2021 climbing higher and brightening as an evening object, and it will have its next encounter with the moon in the evening sky on May 12.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Follow us on Twitter @Spacedotcom and on Facebook.
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