Quasi

Mercury just fails to kiss Venus.

The instant when they will be closest is January 11 at 1 hour Universal Time, which is 1 AM in Britain but by American clocks is back in Jan. 10: 8 PM in the Eastern time zone, 5 PM on the west coast. But the planets will appear almost as close over a span of hours. At any rate it’s something to look for in the after-sunset sky of Saturday Jan. 10, whether you are in America or Europe.

Here is what is happening in space:

Paths of the three inner planets in January, seen from ecliptic north.  The blue line is the sight-line from Earth past Mercury to Venus at Jan. 11 1h UT.

The two innermost planets have both been (from our point of view) swinging out from beyond the Sun. (Venus was behind the Sun last Oct. 25, Mercury on Dec. 8.) So we see them to the “left” (east) of the Sun, in the evening sky. Each evening, there they have been, Venus above Mercury, both gradually getting higher.

Mercury on its nearer, smaller, faster orbit was appearing to move faster, catching up with Venus; but then, as its orbit curved toward us, Mercury appeared to slow. The gap between the two narrowed with increasing slowness, and now what happens is that it reaches a minimum, of only 39 arc-minutes (0.65 of a degree) – hardly more than half the width of your little finger at arm’s length.

Paths of Mercury and Venus on the map of the sky.

This event is an “appulse,” a closest approach of two bodies, but it never reaches the status of being a “conjunction,” which, strictly, is a moment when one body passes exactly north of the other. The phenomenon of an appulse without a conjunction has been called a “quasi-conjunction.” Jean Meeus, who I think invented the term, lists, on page 48 of his Astronomical Tables of the Sun, Moon and Planets, 19 quasi-conjunctions of planets between 1996 and 2020 that are closer than 5 degrees. This is one of the closest, and is the only one between July 2012 and February 2016 – when Mercury and Venus will re-enact this performance of theirs, though coming no closer than 4 degrees.

The two planets appear about 18 degrees from the Sun, but in a slanting direction (depending on your latitude). If you look too soon after sunset, you may well be able to spot Venus – if it’s the only star visible it’s certainly Venus! – but the sky will be too bright for Mercury to show. By about three quarters of an hour after sunset you may be able to find them both. Much longer, and they’ll be getting too low.

The view for latitutde 40 degrees north.  Ticks along the horizon are at 5-degree intervals, so you can see that the planets are at this time about 7 degrees high.  Arrows show the moving bodies’ change of position (relative to the stars) over 5 days.

Mercury’s distance from us is 1.08 astronomical units (Sun-Earth distances), or about 162 million kilometers; Venus’s, 1.58 AU (236 million km). Mercury is shining at magnitude -0.7, Venus -3.9; which in terms of actual luminosity means that Venus at this time appears about twenty times the brighter. To the naked eye they may look at first glance like one brilliant star. But in binoculars or telescope you would see that the two are by no means on top of each other. The apparent widths of Mercury and Venus are only about 6 and 10 seconds, tiny compared to the 39-minute or 2340-second gap between them.

A celestial body that happens at this time to be nearly in the same direction as the planet-pair, but is so far and dim that you will be able to see it only in your mind, is periodic comet Churyumov-Gerasimenko. This is the comet around which the Rosetta spacecraft went into orbit last August, sending its Philae lander down to the surface on November 12.

After the appulse of the two planets, both will at first continue to slide higher into the evening sky, but with Venus maintaining and then rapidly opening out its lead. Mercury will be at its greatest elongation (angular distance from the Sun) on Jan. 14; “stationary” (ceasing to move eastward relative to the starry background) on Jan. 21; then will curl around between us and the Sun on Jan. 30.

Quasi was in Latin a conjunction (in the grammatical sense) meaning “as if” (derived from quam si), and in English became a prefix meaning “like but not really the same as.” Pronunciations range all the way from Latin-like {‘kwasi} to extreme-anglicizing {‘kweizai}.  A quasi-orange would be something that you might take for an orange but isn’t truly an orange.  I leave you to imagine what a quasi-activist would be or a quasi-lover.