You are here: Home USNO News, Tours & Events Sky This Week The Sky This Week, 2018 April 10 - 17
The USNO websites,,,,, and are undergoing modernization efforts.  The expected completion of the work and the estimated return of service is Summer 2020.

The Sky This Week, 2018 April 10 - 17

An elusive Moon, Globe at Night, and Dark Sky Week begins!
Crescent Moon setting, 2018 January 20
HDR composite if the waxing crescent Moon, imaged from Alexandria, VA on 2018 January 20,
with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor and a Canon EOS Rebel T2i DSLR,
HDR composite of four exposures.

The Moon plays hide-and-seek with the Sun this week.  New Moon falls on the 15th at 9:57 pm Eastern Daylight Time.  You’ll find Luna low in the southeastern sky in the gathering light of Dawn as she moves through the dim stars of the rising autumnal constellations.  When she returns to the evening sky oat dusk on the 17th you’ll find her slender crescent about five degrees to the left of brilliant Venus.

We continue the observance of Global Astronomy Month with a reminder to the monthly citizen-science Globe at Night observing campaign, which runs through the evening of the 15th.  Participation is easy: find the constellation of Leo, the Lion in your local nighttime sky, compare your view to the star charts on the Globe at Night website, and report your observation.  You will be helping the astronomical community identify the intensity and distribution of artificial light around the world.  It’s simple to participate, and we encourage you to continue reporting your observations throughout the year.  April 15th is also the beginning of International Dark Sky Week, created 15 years ago by a high-school student to raise awareness of the wonders of the night sky unencumbered by artificial night lighting. International Dark Sky Week draws attention to the problems associated with light pollution and promotes simple solutions available to mitigate it. 

As we mentioned above, Leo is the target constellation for your Globe at Night observations.  You’ll find the Lion and his brightest star Regulus just to the east of the meridian at the end of evening twilight, which now falls shortly after 9:00 pm.  Regulus is a multiple star system that consists of a bright primary and three companion stars.  Two of these stars are visible in backyard telescopes, while the third companion is very close to the primary.  Its existence is betrayed by the spectrograph, which indicates tan orbital period of about 40 days.  Regulus A, the bright component, is a very unusual star in its own right.  Its spectral signature indicates that it is rotating with a period of just 16 hours.  In contrast the Sun rotates once in about 27 days.  This rapid spin means that Regulus is extremely flattened at the poles; it would appear quite elliptical if we could see it up close.  The entire Regulus system is located about 79 light-years from us, which means that the primary shines with about 288 times the luminosity of our Sun.  Regulus forms the bottom of a “backwards question mark” asterism that outlines the Lion’s head.  The next brightest star above Regulus is Algieba, which is a beautiful close double star treat for the small telescope consisting of a pair of golden luminaries closely spaced against the darkness.

To the east of the Lion’s head you’ll find a right triangle of stars that make up his hindquarters.  The star at the eastern acute angle is Denebola, and as the night grows later you can make a large triangle with Denebola, the bright star Arcturus high in the east, and blue-hued Spica to the southeast.  From a dark sky this is a telescope owner’s “happy hunting ground”.  Gazing in this direction we’re looking directly out of the disc of our galaxy, the Milky Way, and or sight line gazes into intergalactic space.  On clear nights you’ll find dozens of faint smudges of light that betray other galaxies that are dozens of millions of light-years from us, a true voyage into “deep space”.

Venus continues to climb higher in the west each night during the fading evening twilight.  She now sets after the end of twilight, so you should have no trouble spotting her in the darkening sky.

Jupiter now rises before 10:00 pm and can be sighted low in the southeastern sky by 11:00 pm.  The giant planet transits the meridian at around 3:00 am, so you still have to wait up until the wee hours or rise well before the Sun to get a good look at him.  However, he’ll make steady progress into the night sky, and in a month he should be visible all night long.

Mars and Saturn are still fixtures in the pre-dawn sky, lolling around the top of the “teapot” asterism in Sagittarius.  Mars is now pulling away from Saturn as he continues his progress eastward along the ecliptic.  By the end of the week his ruddy glow will be just over 8 degrees from the yellow-hued ringed planet.  

USNO Master Clock Time
Javascript must be Enabled