Without a doubt February is “The Month of Venus.” Like a celestial sequined showgirl, she stands more than 40° high in the western sky at sunset for much of this month, reaching her most favorable viewing position and greatest brilliancy of 2017, casting its radiance from high above tree lines onto snowy fields in northern lands.
When February begins, Venus does not set until nearly four hours after the Sun. By month’s end that difference has shrunk a bit to three hours. We are beginning a most fascinating period, which will last until May, for observing Venus with binoculars and telescopes. As the planet begins its swing between Earth and Sun, its disk grows dramatically in size but becomes more backlighted and wanes to a thinner crescent. So during an evening apparition, Venus’s disk grows as it nears Earth, but the crescent thins as the Earth-Sun line of sight diminishes.
Put another way, when Venus appears full (or nearly full), its disk appears relatively small. When it’s very near to Earth, its disk appears nearly six times larger, but it now is just a hairline-thin crescent.
The best combination — or maybe we should say compromise — is the time of its “greatest illuminated extent” (GIE): the moment when we get to see the greatest amount of Venus illuminated in a telescope, which also coincides with the time that Venus attains its greatest brilliancy.
This year, the time of GIE comes on the evening of February 16th in the Americas. We can then observe 331 square arc seconds of the disk’s sunlit surface, about 80 square arc seconds more than we could a month earlier and 25 more than we will when Venus reaches a second GIE in the morning sky in May. Venus will appear crescent-shaped, roughly 25-percent illuminated by the Sun.
Even with such a generous expanse of Venus to observe, the gray markings in the planet’s cloud cover remain quite subtle. The best time to inspect Venus with any optical aid is around sunset or even broad daylight, to reduce the glare of the planet’s brilliant disk against the sky background. Note the date when you first detect evidence of Venus’s crescent phase with 7-power binoculars.
Also watch for signs of the mysterious ashen light – a still unexplained illumination that some observers have occasionally notice in parts of Venus’s night side.