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The Sky This Week, 2016 March 15 - 23

The equinox and the calendar.
Jupiter, imaged on 2016 March 9 at 02:45 UT

The Moon brightens the evening sky this week, waxing from First Quarter Full Moon, which occurs on the 23rd at 8:01 am Eastern Daylight Time. The Full Moon of March is popularly known as the Worm Moon, Crow Moon, Sap Moon, or Lenten Moon. This year it is a very important one for believers of Christianity. Since it is the first Full Moon that follows the vernal equinox it fixes the date of Easter, the most important of the moveable feasts in the Christian calendar. Luna starts the week high above the stars of Orion, then wends her way eastward through Gemini and Cancer. You’ll find her just under five degrees southeast of the bright star Regulus on the evening of the 20th. On the following night she cozies up to bright Jupiter, passing just two degrees south of the bright planet.

The vernal equinox occurs on the 20th at 12:30 am EDT. This is the moment when the Sun’s ecliptic longitude reaches zero degrees. For many ancient cultures this moment marked the beginning of a new year, and its date still figures prominently in the calendars of many religions. As mentioned before, it fixes the date of Easter for most of the world’s Christians, and it was the computation of this important date that led to the civil calendar that is almost universally observed today. The Gregorian Reform of 1582 was instituted by Pope Gregory XIII to correct errors in the Julian Calendar and the "Computus", the formula used to calculate the date of Easter. By Gregory’s time the observed date of the equinox had slipped to March 11 in the Julian system, a full 10 days before the ecclesiastical equinox that was defined as March 21. Gregory’s reformers changed the formula for reckoning leap years, changing the Julian formula from a leap year every four calendar years to one in which there were 97 leap years in a 400 year cycle. Next, they removed 10 days from the calendar in October, 1582 to bring the astronomical vernal equinox back in synch with the ecclesiastical date of March 21. The Gregorian Calendar is not quite perfect, though, exceeding the true astronomical year by about 11 seconds. This error slowly accumulates, so equinoxes are gradually occurring a bit before the defined date of March 21. However, the date of the equinox won’t slip by a full day until somewhere around the year 3000.

In addition to the equinox, another sign of spring is now appearing in the east. By 10:00 pm EDT you may notice a bright star appearing above the eastern horizon. This is Arcturus, the brightest star in the northern hemisphere sky and the fourth-brightest of all the stars. The star’s rosy tint contrasts nicely with the cooler blue tints of the bright stars of winter found in Orion and Canis Major. It is one of the closer stars to the solar system, located at around 37 light-years from the Sun.

Thanks to Daylight Time, Jupiter gets a little bit more time to shine in the evening sky. The giant planet spends most of the night above the horizon, transiting the meridian after midnight. This should give you plenty of time to have a long look at this distant world that shows the largest apparent planetary disc to Earth-based telescopes. Virtually any optical aid will show the planet and his four bright moons first described by Galileo in 1610. As you increase the aperture of your telescope, more detail becomes apparent. In my four-inch refractor the planet’s prominent equatorial cloud belts are easily seen, and careful examination reveals swirls and spots within them as well as fainter parallel bands. An 8-inch telescope will show a great amount of fine-structure detail on nights of steady air, and the Galilean moons show discs and distinctive tints that reveal their identities. If you look at Old Jove on the evening of the 17th, you’ll have an excellent chance to see his most famous feature, the Great Red Spot, rotate across the disc.

Mars and Saturn may be found near the meridian in the pre-dawn hours. At 6:00 am they are both ideally placed for viewing through the telescope. Mars shows a gradually growing pink-tinted disc rimmed by a prominent white polar cap. Saturn looks like…well, Saturn. The planet’s distinctive rings are tipped about as much as they can get along our line of sight. Both planets will form an attractive triangle with the bright reddish star Antares for the next several weeks.

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