Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during April 2015.

Orion is sinking in the west as darkness falls, followed by Gemini and its bright stars Castor and Pollux. Further towards the south is the faint constellation of Cancer. It contains the Beehive Cluster, an open star cluster visible in binoculars, and currently plays host to the planet Jupiter as well. Leo is due south in the evening, with its bright star Regulus. Nearby, in Virgo and Coma Bernices, is an area called the Realm of the Galaxies. In this region, an 8" telescope can pick out a number of galaxies that are part of the Virgo Cluster, the largest cluster of galaxies in our local universe and itself part of a much bigger supercluster. Higher up is Bootes, with its bright star Arcturus, and overhead is Ursa Major. Later in the night, Lyra and its bright star Vega rise in the north-east.

The Planets

• Jupiter, two months past opposition (when it was opposite the Sun in the sky), is still high in the south-west in the evening. During April, it dims from magnitude -2.3 to -2.1 and shrinks from 41.5 to 38" across. As it switches from westward (retrograde) to eastward motion this month, it remains in Cancer and moves very little relative to the stars. A small telescope can reveal the equatorial bands, Great Red Spot (at certain times) and Galilean moons.
• Saturn rises in the evening, a little earlier each night, and lies close to the left-hand star of the fan of Scorpius. It brightens from magnitude +0.3 to +0.1 and grows from 17.8 to 18.4" in diameter during the month. It reaches 22 degrees' elevation when due south in the early hours of the morning, and the ring system is inclined at 25 degrees to our line of sight.
• Mercury reaches superior conjunction (behind the Sun in the sky) on the 10th, and remains invisible until around the 19th, when it appears low in the west-north-west about 45 minutes after sunset. Shining at magnitude -1.4, it climbs higher each evening on its way to eastern elongation (its furthest from the Sun in the sky) on the 7th of May.
• Mars, ever-present in the evening sky for many months, is finally disappearing into the Sun's glare. Lying close to Mercury from around the 19th to 24th, it has an angular size of 4" and so reveals no surface details to us here on Earth.
• Venus blazes at magnitude -4, rising higher in the evening western sky as the month progresses. It moves from Aries into Taurus on the 7th, aproaching the Pleiades Cluster around the 13th. Its angular size increases from 14 to 16" during April, as its phase wanes from 78 to 68 percent.

Highlights

• Saturn is near a waning gibbous Moon before dawn on the 9th.
• Venus can be seen near the Pleaides Cluster about 45 minutes after sunset from the 11th to 13th.
• Mars, Mercury and a wafer-thin crescent Moon form a near-equilateral triangle just after sunset on the 19th, with about 4-5 degrees between each of them. With the Moon under a day old and only 13.5 degrees from the Sun, it is only just visible and may require binoculars or a wide-field telescope to be seen. Look up and a little to the left of where the Sun has set, but not until the Sun really has gone down.
• Venus is close to a waxing crescent Moon, just above the red star Aldebaran and the Hyades Cluster in Taurus, in the western sky on the evening of the 21st.
• Jupiter lies about 6 degrees above a first-quarter Moon in Cancer on the 26th.

Southern Hemisphere

Claire Bretherton from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during April 2015

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The night of the 4th-5th sees the first lunar eclipse of the year, and also the backward move of the clocks in parts of the southern hemisphere. In Wellington, New Zealand, the penumbral phase begins at 22:03 NZDT (New Zealand Daylight Time, 13 hours ahead of Universal Time) as the Earth begins to obscure sunlight from the Moon's surface. The umbral phase, when sunlight is fully blocked from part of the Moon, starts at 23:17. The Moon is cast into total shadow for 7 minutes, from 00:57 to 01:04. The umbral phase ends at 02:44, while the penumbral phase finishes at 02:58 NZST (New Zealand Standard Time, 12 hours ahead of Universal Time).

The Moon is in Virgo during the eclipse, about halfway between Jupiter in the north-west and Saturn in the east. Venus appears brightly in the evening, and sets 2 hours after the Sun by the end of the month. Saturn is in Scorpius, a little below the red star Antares. It rises around 22:00 NZDT at the beginning of the month and 19:00 NZST at the end.

The constellation of Centaurus is high in the east after dark, with its two brightest stars, Alpha and Beta Centauri, pointing towards Crux, the Southern Cross. Centaurus is one of the largest constellations and contains many bright stars, clusters and nebulae. The globular cluster Omega Centauri, the largest and brightest in the Milky Way Galaxy, appears similar in size to the full Moon when seen with the naked eye at magnitude +3.7. Binoculars reveal individual stars and a dense core. With a population of stars that are around 12 billion years old, it is a relic of the early Universe and may be the remnant of a dwarf galaxy that merged with the Milky Way. NGC 3766 and NGC 5460 are two open star clusters in Centaurus, both just visible to the naked eye. The planetary nebula NGC 3918, the 'Blue Planetary', is also located there, and, at magnitude +8, its blue oval shape can be seen with a small telescope. Centaurus hosts one of the closest galaxy clusters to Earth. Separately, it is home to NGC 5128, the galaxy known as Centaurus A. Centaurus A is elliptical, but has a dark dust lane across the middle, and the supermassive black hole at its heart is thought to be consuming a spiral galaxy with which it has merged. As a result, it emits relativistic jets that can be detected at radio and X-ray wavelengths. Located less than 5 degrees from Omega Centauri, Centaurus A is the fifth-brightest galaxy in our sky (excluding the Milky Way) and is easily visible in binoculars. The bright central bulge and dark lane may be viewed with larger binoculars, while a telescope reveals more of the galaxy's structure.