You are here: Home USNO News, Tours & Events Sky This Week The Sky This Week, 2018 March 13 - 20

The Sky This Week, 2018 March 13 - 20

Spring is (finally!) here.
Mercury & Venus, 2018 March 5, imaged at the USNO
Venus and Mercury imaged from Washington, DC on 2018 March 5, 23:50 UT
with a Canon EOS Rebel T2i DSLR, 250mm @ f/6.3, 1/80s, ISO 3200.

The Moon plays hard-to-get this week, spending most of her time in the glow of morning and evening twilight.  New Moon occurs on the 17th at 9:12 am Eastern Daylight Time.  Luna returns to the evening sky on the 18th, when you’ll find her sliver of a crescent low in the west about 45 minutes after local sunset.  In addition to the Moon, bright Venus should also be visible just under four degrees to the right of the crescent.  You should also look for the planet Mercury, which will be a similar distance above and to the right of Venus.  You may need binoculars to sight Mercury at first, but as the sky darkens the fleet planet should become visible to the naked eye.  This should present a very nice twilight photo opportunity!

The vernal equinox occurs on the 20th at 12:15 pm EDT.  This is the moment when the center of the Sun’s disc reaches an ecliptic longitude of zero degrees.  At this time the Sun crosses the celestial equator at a point directly over northern Brazil.  The term “equinox” implies that the length of daytime and nighttime are equal on this date, but here in Washington we see 12 hours of day and night on the 17th.  This is due to the fact that the Sun subtends a disc about half a degree in diameter, so its limbs cross the horizon well before the disc center does.   From the 17th until September 25th the days will be longer than nights.

We are currently in the middle of the March campaign to count stars for the Globe at Night citizen-science campaign.  We’re currently using the stars in the constellation of Leo, the Lion, for our estimates of the darkness of the sky.  Leo’s brightest star, Regulus, may be found rising in the eastern sky as twilight ends, and by 10:00 pm the entire constellation is well up in the east.  The main stars of Leo’s outline range from first to fourth magnitude, so you have a good range of stars to look form.  Compare your observations with the star charts available on the Globe at Night website and help scientists map out the spread of artificial light pollution.

Venus and Mercury continue their dance in the western twilight sky.  Mercury will reach his greatest elongation east of the Sun on the 15th.  At this time he will be just 18 degrees from the Sun, but because of his orbital geometry he’ll be at his most favorable placement in the evening sky for the year.  He spends the week within a few degrees of Venus, but during the course of the week he fades from -0.3 magnitude to 1.2, a factor of four times fainter.  To find him locate Venus in the evening twilight about half an hour after sunset.  Use binoculars to search for Mercury above and do the right of Venus.  Once you’ve found him you should be able to spot him with the naked eye about 45 minutes after sunset.  By the end of the week he’ll be a difficult naked-eye target, but he should still be visible in binoculars.  As a bonus, the 36-hour old crescent Moon will be just to the left of Venus at dusk on the 18th.

I’ll be honest and admit that I’m not a big fan of Daylight Time.  It pushes the time I can set up my telescope for evening observing much closer to my bed time.  However, it does offer one positive in that sunrise is now around 7:00 am.  This means that we don’t have to get up too early to enjoy the planets that are hanging in the morning sky.  Jupiter, Mars, and Saturn are strung like beads along the ecliptic over the southern horizon as morning twilight approaches.  Jupiter is the brightest of the trio, just west of the meridian in the dim constellation of Libra, the Scales.  Mars follows in the southeast, his ruddy glimmer making a nice contrast with yellow-hued Saturn.  Mars starts the week about 10 degrees west of Saturn.  By the week’s end he’s moved four degrees closer to the ringed planet.

USNO Master Clock Time
Javascript must be Enabled