April 2015

Sun and Moon

FULL MOON occurs on the 4th April at 14:05 PM. LAST QUARTER (half moon in the morning sky) falls on the 12th. NEW MOON is on the 18th at 20:57 PM. FIRST QUARTER (half moon in the evening sky) falls on the 25th.

On the 18th April the young lunar crescent will not be visible from Earth. On the 19th, the crescent will be visible over North, Central and South America, Western and Central Europe, the Middle East and Africa. It may be visible with optical aid from parts of Asia and Indonesia on the 19th. 

On the 1st April at 15:00 PM the Moon will be at apogee (furthest from Earth) at a distance of about 406 011 km. The Moon will be at perigee (closest approach to Earth) at a distance of about 361 025 km on the 17th at  05:54 AM.

On the 4th April there will be a total lunar eclipse. The eclipse starts at 11:01 AM and finishes at 16:59 PM SAST and will be visible over most of North America, South America, Asia and parts of Australia. Unfortunately, the eclipse will not be visible from South Africa.

Planetary and Other Events – Morning and Evening

Mercury is visible in the early morning for the first week of the month before passing behind the Sun second week. Once Mercury emerges from the Sun’s glare it will be visible in the early evening for the remainder of the month. Venus is visible as a bright early evening star, setting around 20:15 PM all month  (all times quoted for Cape Town). Mars is also visible in the early evening twilight all month, but is fainter than Venus and sets earlier.

Jupiter is visible from dusk until around 02:20 AM at the beginning of the month, although by month’s end it will set by 00:30 PM. Saturn rises after 21:30 PM at the beginning of the month (around 19:30 PM by month’s end) and is up all night. Uranus and Neptune are both too faint to observe with the naked eye, but may be seen using binoculars or a small telescope. Uranus is too close to the Sun to observe in the first half of the month, but as it emerges from the Sun’s glare it will be visible in the early morning close to the Sun. Neptune rises early morning, around 04:20 AM at the beginning of the month, rising progressively earlier and earlier each night, until at month’s end it rises around 02:30 AM.

Four meteor showers are active in April, the delta Pavonids, April Lyrids, pi Puppids and the eta Aquariids. Observing prospects for the delta Pavonids and eta Aquariids are poor due to bright Moon around their peaks. Observing prospects for the April Lyrids are good and they are visible from the 16th –  25th  April, peaking in the early hours of the 23rd. They are best viewed between 02:00 AM and 05:00 AM looking towards the constellation of Lyra. The maximum hourly rate is expected to be around 15. The pi Puppids are active from the 15th – 28th April, peaking on the night of the 23rd. This shower is best viewed between 19:00 PM and 23:00 PM looking towards the constellation of Puppis.

The Evening Sky Stars

The bright stars near the summer Milky Way are still prominent in the sky in April evenings, but are further to the west than in March. Stars rise about 2 hours earlier and set about 2 hours earlier than they did on the same day of the previous month — our point of view keeps changing as Earth goes around the Sun. The Milky Way gets more and more impressive as the night progresses, as the centre of our galaxy rises in the east and eventually passes overhead before dawn.

Orion can still be seen in the NW in early evening, with Taurus the Bull charging at him from low in the NW. Since Orion is upside down for us in the southern hemisphere, we see Sirius (brightest star in the sky and the eye of Orion’s Large Dog) ‘above’ Orion, with the dim stars of the Hare just above Orion’s feet. Between Orion and the stars of Leo the Lion (rising in the NE in early evening) are the dim stars of the Unicorn and the Crab, with Procyon (brightest star in Orion’s Small Dog) between them, while below Procyon are the ‘Twin’ stars Castor and Pollux low in the NNW.

In the NNE at the end of evening twilight is the upside-down question mark of Leo the Lion, representing the Lion’s head and mane. Brightest of Leo’s stars is Regulus, the ‘prince’ and one of the four ‘royal stars’, Second-brightest among Leo’s stars is Denebola (‘tail of the lion’), well to the east (right, for an observer facing north) of the ‘question mark’. According to Egyptian legend, the sun was in Leo immediately after the Creation, near Denebola. On a more scientific note, Denebola is about 36 light years away, and 12 times as bright as our sun. The dust clouds around this star suggest that planets may have formed around it, but none have been detected yet.

Blue-white Spica, brightest star in Virgo, is often shown as an ear of wheat held by a young woman, who in Babylonian mythology was queen of the stars and the lover of the god of grain. Above her in the eastern sky fly the four stars of the Crow. The head of the water snake (Hydra) is a small group of stars between Leo and Orion’s Big Dog. The rest of the snake winds its way down toward the Crow, with its brightest star, Alphard, heart of the serpent, high in the northwest. Above and to the left of the Crow, for an observer facing east, is the Cup. Alphard is an Arabic name meaning the ‘solitary one’, as there are no other bright stars near it. At about 40 times the diameter of the sun and 400 times as bright, Alphard is one of the ‘bright giants’ in our neighbourhood. But our ‘neighbourhood’ is rather large. Alphard is 11 million times as far away from us as our own sun – so it looks a lot dimmer to us!

To the south of Sirius, and nearly overhead, is Canopus, second brightest star in the sky, lighting the keel of the great ship Argo. If they were at the same distance, Canopus would appear far brighter than Sirius. Canopus is 15000 times as luminous as the sun, a rare yellow-white supergiant 313 light years away. If placed at the center of our solar system, its surface would be three quarters of the distance from the centre to the orbit of Mercury, Earth would be global toast, and so would we.

Rising in the southeast in April evenings are the stars of the Southern Cross and the Pointers (Alpha and Beta Centauri). The Alpha Centauri seen by the eye as a single bright star is easily separated by a small telescope into a brilliant pair of yellow stars, Alpha Centauri A and B. The brighter one is about half again as bright as our sun, the fainter about half as bright as the sun. Separated by about 23 times the distance from the Earth to the sun, they take 80 years to orbit each other. For many years it was believed that these were the two nearest stars to our own sun. But a third star also appears to be a member of the Alpha Centauri system, and is slightly closer to us (4.22 vs. 4.35 light years). This star, ‘Proxima Centauri’, was discovered in 1915 by Robert Innes of the Union Observatory in Johannesburg. It’s about 1/7 of the mass and diameter of the sun, and so dim that even as seen from Alpha Centauri A and B, it would be only dimly visible to the naked eye. If Proxima is in orbit around stars A and B, it probably takes about 500 000 years to go around once.

Achernar (the Little Horn) shines in the southwest in early evening, with the Magellanic Clouds nearby. These dimly glowing patches (actually nearby galaxies) were seen by some African peoples as hunting grounds for the gods, and their visibility was used to predict the weather. To the extent this worked, it probably related to the amount of dust in the air.

The Morning Sky Stars

Achernar shines brightly in the SE before dawn, with the celestial river Eridanus flowing down to the SE horizon. The Crane, the Phoenix, the Toucan, the Bird of Paradise and the Peacock are all in neighbouring parts of the southern sky, while the Sea Serpent is also in the vicinity. The stars of the Pointers and the Cross are still visible low in the SW, with the stars of the Altar, the Level and the Wolf lying between the Pointers and the Scorpion.

Almost overhead in the predawn sky at the beginning of the month is the centre of our Milky Way galaxy, with the constellations of the Archer and the Scorpion. The dark patches in the Milky Way are dust clouds that block the light of the stars behind them. It’s in the heart of dark clouds like these that new stars are constantly being born. The Khoikhoi and San said the Milky Way came into being when a strong-willed girl became angry and grabbed the roasting roots (of which her mum did not want to give her any) from the fire and threw the ashes and roots into the sky. The red and white roots now glow as red and white stars and the ashes are the Milky Way.

High in the east is Fomalhaut, brightest star in the Southern Fish, with the dimmer stars of the Sea Goat and the Water Carrier just to the north. Further to the north we get more birds. Fairly high in the north is Altair, brightest star in the Eagle and the 11th brightest star in the sky, while very low in the N to NE is the Swan, with the bright star Deneb just above our horizon. Low in the northern sky is brilliant Vega, about 25 light years from us and accompanied, as with Denebola and a number of other youngish stars, by a disk of dust which may be a leftover from the formation of a system of planets.

Nicola Loaring   30 March 2015

What's Up

PDF version (two pages, including the full text)
Comments are closed.