The Evening Sky in April 2015
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The bright planets Venus and Jupiter light up the twilight sky. Venus is in the northwest, brilliant and silver. Golden Jupiter is in the north. Venus sets before 8 pm NZST. As Venus sets Saturn rises on the opposite horizon. It isn't eye-catching like Venus and Jupiter but is the brightest 'star' low in the southeast sky.
On the night of April 4-5, Easter Saturday-Sunday, there is a total lunar eclipse. The moon enters the penumbra, the outer part of Earth's shadow, just after 10 pm NZDT. It will slowly darken on its right side. A more obvious darkening begins around 11:16 when it begins to move into the centre part of the Earth's shadow, the umbra. It is only just into the umbra at 1 a.m. when it begins to move out again. It is clear of the umbra at 2:45 a.m. NZDT which is 1:45 NZST if you have reset your clock. The moon is out of the penumbra at 3 a.m. NZST. This eclipse is the minimum that can be counted as a total eclipse. The moon is in the umbra for just five minutes: 12:58 to 13:03. So it should remain quite bright on its lower edge.
A small telescope will show the disk of Jupiter with its four bright 'Galilean' moons lined up on each side. Binoculars, held steady, will sometimes show one or two moons looking like faint stars close to the planet. Jupiter is 740 million km away mid-month. It sets in the northwest around midnight.
Venus, though bright, is small and featureless in a telescope like a tiny gibbous moon. It is catching up on Earth from the far side of the sun. As it does so it will increase its angle from the sun, causing it to set later in the night. At the end of the month Mercury might be glimpsed setting in the early twilight below and left of Venus. It sinks back into the twilight in May as it passes between us and the sun.
Saturn rises about 10 pm NZDT at the beginning of April; around 7 pm NZST by month's end. It is just below a curve of stars making the Scorpion's claws. Orange Antares is to the right of Saturn and fainter. A small telescope shows Saturn as an oval, the rings and planet blended. Larger telescopes separate the planet and rings and may show Saturn's moons looking like faint stars close to the planet. Titan, one of the biggest moons in the solar system, orbits about four ring diameters from the planet. Saturn is1370 million km away mid-month. The Moon will appear close to Saturn on the night of April 8-9.
Sirius is the first true star to appear at dusk, midway down the northwest sky. It is soon followed by Canopus, southwest of the zenith. Below Sirius are Rigel and Betelgeuse, the brightest stars in Orion. Between them is a line of three stars: Orion's belt. To southern hemisphere star watchers, the line of three makes the bottom of 'The Pot', now tipped on its side. Below and right of Sirius is Procyon.
Just left of Jupiter is a fuzzy patch of light, the Praesepe cluster, marking the shell of Cancer the Crab. Praesepe is also called the Beehive cluster, the reason obvious when it is viewed in binoculars. Lower and further left are Pollux and Castor, the heads of Gemini the twins, making a vertical pair.
Crux, the Southern Cross, is high in the southeast. Below it, and brighter, are Beta and Alpha Centauri, often called 'The Pointers'. Alpha Centauri is the closest naked-eye star, 4.3 light years (l.y)* away. Beta Centauri, like most of the stars in Crux, is a blue-giant star hundreds of l.y. away. Canopus is also a very luminous distant star; 13 000 times brighter than the sun and 300 l.y. away.
The Milky Way is brightest in the southeast above Crux. The Milky Way can be traced to nearly overhead where it fades. It becomes very faint in the northwest, right of Orion. The Milky Way is our edgewise view of the galaxy, the pancake of billions of stars of which the sun is just one.
The Clouds of Magellan, LMC and SMC are midway down the southwest sky, easily seen by eye on a dark moonless night. They are two small galaxies about 160 000 and 200 000 light years away.
*A light year (l.y.)is the distance that light travels in one year: nearly 10 million million km or 1013 km. Sunlight takes eight minutes to get here; moonlight about one second. Sunlight reaches Neptune, the outermost major planet, in four hours. It takes four years to reach the nearest star, Alpha Centauri.
Notes by Alan Gilmore,
University of Canterbury's Mt John Observatory,
P.O. Box 56,
Lake Tekapo 7945,