The Spring Show

Saturday April 25 is Astronomy Day, so here is the western sky as night falls this evening.

There are Astronomy Day star parties and other events at many places around the U.S., such as museums, planetariums, and clubs. You can find information on them at www.astroleague.org/al/astroday/astrodayform.html.

Our picture is for latitude 40 north and longitude 75 west – Philadelphia – but it scarcely differs for other places in the U.S. or around the world’s north-temperate latitudes.

Astronomy Day started in 1973. It is held in April or May on the Saturday closest to the First Quarter Moon. The reason for not choosing instead a date close to New Moon is presumably that the Moon itself is an interesting sight, especially when the terminator (the light-dark boundary) is half way across it and revealing lunar mountains in high relief; but moonlight is not yet glaring enough to obliterate the sky’s other sights.  And, this year, the instant when the Moon is at First Quarter happens to be April 25 at 23:55 by Universal Time – close to midnight between the 25th and 26th. By Eastern U.S. clocks this is 7:55 PM on the 25th – about half an hour before the time of our picture. And there is the First Quarter Moon, high up over the southwestern horizon, in Cancer and perhaps not even drowning out the wonderful star cluster called the Beehive or, alternatively, Praesepe, the “manger.” The winter” constellations – those around Orion, high in winter evenings – still show, though tilting down these evening to the west. Mercury and Mars, low to the horizon, are hard to discern, but Venus continues its climb toward the peak it will reach in June.

Imagine the Moon as an impresario, brilliant though modestly half-hiding his brilliance so as not to outshine his cast: he throws aside the curtain to reveal this starlit scene: “Ladies and gentlemen – Astronomy Day! Or, rather, Astronomy Night! Raise your opera glasses and behold our performers!”

This is Spring Astronomy Day, because from 2008 there has also been a Fall one, which will this year be on September 19.