April 2009 Night Skies over Tanzania
The 100 Hours of activity of the International Year of Astronomy is with us and many of you will be participating in looking up at the skies to make sense of all the jumble up there. This activity is being conducted all over the world during the 4 days (hence 24 x 4 = 96 which is approximately 100 hours) from 2nd to 5th April 2009. Its aim is to allow millions of people to be amazed by all the wonders that lie hidden in the jumble that we see above us. Visit http://www.100hoursofastronomy.org for details on the various activities, some of which you can follow on the internet. Read also about the International Year of Astronomy at http://astronomy2009.org/
How do we make sense of the sky; how can we find our way in the sky? These are questions that seem to perplex newcomers to astronomy. It can be acknowledged that it does take quite an effort to understand the sky despite the immense interest in the mysteries that stars arouse. The best way to enjoy astronomy is to join an astronomy interest group such as an astronomy club of society. Easily recognizable constellations can then be pointed out to you one at a time as you get used to the pathways in the sky, just as you get used to finding your way around when you visit a new city.
However we can also learn to find our way in the maze of stars using a sky map. We can do this by focusing our attention on a small region of the heavens that can be easily identified from the star map. Important constellations, stars and their characteristics can be revealed as you slowly make sense of the whole sky and eventually become aware of the mysteries of the Universe. Eventually after most of the prominent regions of the sky have been covered, the whole sky map will also be understood.
This month we shall concentrate on a region of the sky that is unique to the southern hemisphere. The spectacle unfolds slowly as the night progresses while facing southwards slightly east of south. Two bright stars rise vertically straight up from the horizon in the early evening. The line connecting the two stars points up towards a well-known group of stars shaped in the form of a kite lying on its side. This kite like group is otherwise known as the famous Southern Cross or the Crux constellation, marveled by early northern seafarers and a welcome beacon that pointed the way south in an otherwise unknown sky.
The bright vertical pair of stars is known as “the pointers” because although the line connecting them becomes gradually horizontal as the night progresses, they continue to point towards the Southern Cross throughout the night. The pointers are part of the Centaurus constellation named after the legendary centaur with the head and torso of a human and the body and legs of a horse.
The lower and brighter of the two stars of the pointers is called Alpha Centauri to mark it as the brightest stars in the Centaurus constellation while the lower one is called Beta Centauri, that is the second brightest star in that constellation. Of the two, Alpha Centauri is the most interesting because it is the fourth brightest star in the whole sky and has the common name Rigel Kent. Next to the Sun, it is the nearest star at 4.3 light-years which means that its light, travelling at 300,000 kilometers per second takes more than 4 years to reach us. In comparison, light takes only a second to reach us from the Moon and about 9 minutes from the Sun, which is nearly 150,000,000 kilometers away. When view through a telescope it shows up as two stars because it is a multiple system of stars.
The sky map this month shows “the pointers” at 9 p.m. pointing towards the Southern Cross (or Crux) that looks like a kite. They are already leaning right with the diagonal of the kite pointing south. Earlier it would be lying horizontal and parallel to the earth so cannot be used to show south direction since the actual south point is almost 6 degrees above the southern horizon since we are at latitude 6 degrees south. There is no prominent star that marks the south point similar to the north-star Polaris that marks the celestial north pole. The map shows how to get the south point by marking the crossing of the line connecting the diagonal of the kite observed two hours apart. As the night progresses the kite of the Southern Cross appears more and more vertical and even leans the other way, due to rotation of the earth about the south celestial pole. As the days go by, especially if you look at the southern skies after at least two weeks you will also see that the kite appears more and more vertical even in the early evening. This is due to the gradual movement of our Earth in its orbit around the Sun.
Among the planets, Saturn is the only one clearly visible but appears as a very sharply shining point which can be identified by its steady light compared to the twinkling of nearby stars. It shines half way up the eastern horizon. On 6th April the moon will be close to Saturn. Through a telescope Saturn shows its ring as a thin line, since the plane of the flat ring of rocks surrounding the planet is almost along our line of sight. Saturn’s largest moon Titan can be seen a point when viewed through the telescope.
Mercury which travels fast around the sun will make its appearance in the evening skies from 11th April after which it will be more than 10 degrees above the western horizon at sunset and reaches 15 degree elevation by 25th April. Due to its small size, Mercury is difficult to make out so you will have to take extra care to locate it. On 26th April, a thin crescent moon will be close to it so you can try to trace the planet which is in a half phase when viewed through a telescope, though you would need quite a powerful telescope to view that.
The phases of the Moon this month will be: 2nd April – First Quarter, 9th April – Full Moon, 17th April – Last Quarter and 25th April – New Moon. Have you wondered what makes the moon change its shape? Imagine a ping pong ball that is half black and half white. Hold the ball in between the first finger and thumb with the black side facing you. Now turn the ball slightly so that on a small part of the white surface is visible. Lo and behold –a crescent appears! Turn the ball further and half is visible white and half black. This is the first quarter phase. Turn further and the whole white hemisphere will be facing you, which is equivalent to the full moon phase. The white side can be taken to be the side on which the sun shines on the moon while the dark side is the night side of the moon. When looking through a telescope, the moon shows breathtaking details of its craters when it is in the first quarter phase (around 2nd to 5th April). This is because the at the edge between the white and black parts the sun is shining at a low angle (i.e. it is sunrise or sunset time on the moon) hence very long shadows are casts which our eyes make out as a relief of the craters on the moon.
Venus reappears this month as a very bright morning star from 4th April onwards, shining with magnitude -2.1. It reaches 35 degree elevation above the eastern horizon at sunrise by the end of the month. For several days from 20th April, Venus and Mars will be seen near each other, while on 22nd April, the pair will be joined by a crescent moon in the eastern horizon just before dawn.
Jupiter is in the morning sky shining very brightly, and on 20th April the moon will be close to it.
Enjoy the skies and let your mind wander around the night skies in wonder.