The Night Sky this Month

March 2010 by Russell Eberst

The spring equinox (for the northern hemisphere) arrives on March 20. The Sun crosses the celestial equator on March 20 at 17.32 U.T. going northwards, marking the exact equinox. "Equinox" indicates that the length of the day and night are expected to be equal on that date.

If the tables of sunrise and sunset are inspected, it will be found that at the equinox, the length of daytime exceeds that of night time by a few minutes. This is due to the refractive effect of the Earth's atmosphere which raises the apparent position of the Sun at sunrise and sunset by almost a degree, advancing the time of sunrise and delaying the time of sunset. There is also the effect caused by the times of sunrise and sunset being measured when the upper limb (or edge) of the Sun is on the horizon, and not the centre of the Sun.

The actual date on which there are twelve hours of daylight and twelve of night is nearer March 18. The rapid change in the time of onset of the evening dusk means that the familiar stars of winter move quickly towards twilight and to being lost in the Sun's glare. Nevertheless, the prominent winter stars such as Orion and its surrounding assemblage will remain visible throughout March.

The ecliptic which marks the paths of Sun, Moon and planets among the stars, reaches its highest angle relative to the western horizon, in the evenings during March. This means that planets such as Mercury and Venus are positioned more favourably than at any other time of the year. Venus is slowly emerging from behind the Sun, having reached "superior conjunction" on January 11. It has already been spotted by amateurs as it extends its separation from the Sun, to be 19° by the end of the month. This should make it reasonably easy to spot in the western sky, low down as the sky darkens. It sets an hour after the Sun at the beginning of March, and almost two hours after the Sun at the end.

It will be accompanied by the thin crescent Moon on the evening of March 17. The phenomenon of Earthshine (also known as 'the old Moon in the young Moon's arms) should be visible on that evening and a few subsequent evenings. The planet Mercury passes through superior conjunction on March 14, emerging into the evening sky, chasing Venus moving eastwards.

Mercury fails to catch Venus but gets to within 3°.5 of the brighter planet by month's end, so will appear within the same binocular field. This provides a rare opportunity of seeing Mercury, which often proves elusive due to its proximity to the Sun. First locate Venus, and place it in the upper-left section of the binocular field-of-view. The dimmer Mercury should be seen in the lower-right section.

The poor view that Earth-based observers get of Mercury will be superseded by the close orbit view to be obtained by the Messenger spacecraft, when it enters orbit around Mercury in March 2011, just a year away.

Jupiter went through superior conjunction on the last day of February, so will slowly emerge into the morning sky. The low ecliptic angle will mean it will be lost in the dawn twilight for much of the month. The most prominent planets this month are Mars and Saturn. Although Mars is now well past its opposition and closest to us, it is still obvious even to casual observers. It is located in western Cancer, not far from the twins - Pollux and Castor. It has been moving westwards, or retrograding, as the Earth speeds past it on our inner orbit. During March, the retrograde motion decreases, coming to a halt at a stationary point on March 11. By then it will be found about halfway between Pollux and Praesepe (an open cluster, also known as M44).

The second half of March will see Mars resume a prograde (eastwards) motion in the direction of Praesepe. Throughout the month its distance will be increasing, leading to a steady decrease in brightness. Its magnitude on March 1 is -0.6, brighter than any nearby star (with the exception of Sirius), but by March 31 it is down to magnitude +0.1, comparable with Capella or Rigel. The size of its observable disc similarly diminishes; giving observers only limited views of details on the red planet.

Very steady atmospheric conditions will be needed to distinguish these details at a distance of 1 A.U. from Earth, reached on March 31. An astronomical unit (A.U.) is 93 million miles, or 150 million km.

The ringed gas giant planet Saturn reaches its best and nearest for this year when it comes to opposition on March 22. It lies in the constellation of Virgo, the virgin. Saturn retrogrades from the beginning of February until the end of May, moving on a track just north of a line joining eta Virginis to beta Virginis (Zavijah). Its magnitude is about +0.5, so brighter than any nearby star, and perceptibly brighter than Spica, which is the brightest star in Virgo.

Saturn's rings are seen close to edge on, so they don't add much to the total brightness of Saturn. The spacecraft Cassini which has been orbiting Saturn since July 2004 extends its extremely successful mission with a close flyby of Rhea, a crater-pitted rocky moon on March 2. Cassini will get to within 101 kilometres (63 miles) of the surface, and will provide some amazing views of this distant satellite.

Talking of satellites, the largest man-made satellite, the International Space Station (I.S.S.) is nicely placed for U.K. observers during March. There are evening passes from March 4 through to March 20.

From Scotland many of its passes will take it through the constellation of Orion, and observers will get an opportunity of comparing its brightness with that of Sirius, the dog star, which is in the neighbouring constellation of Canis Major. Many others of the 15,000 man-made objects in orbit will also be visible to the unaided eye, with even more detectable using ordinary binoculars.

At the end of March, civil time will once again revert to British Summer Time (B.S.T.). Early on the morning of Sunday March 28, clocks and watches should have their time advanced by one hour.


Moon phases

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