By Dr Noorali T. Jiwaji
ntjiwaji at yahoo dot com
CLICK ON THE LINKS BELOW THE SKY PICTURES TO GET LARGER VIEW
There is an amazing show going on at the moment in the western sky soon after sunset, with two very bright planets, Venus and Jupiter, SHIFTING their positions very distinctly day after day, from now until the end of November and beginning December. Every day, watch the sky soon after sunset and you will see Venus as a very bright star closer to the horizon in the west. Slightly less bright Jupiter is 15 degrees above Venus and the two are separated by about two palm widths. The two planets show very distinct movement when watched over several days. Venus edges up towards Jupiter over the course of this month, the two finally meeting up at the end of the month like two jewels in the sky.
Follow the pictures of the evening sky as seen just after sunset,
to see how the two planets will show a very noticeable shift in their relative
positions. They are very far apart at the moment (1), but over the next 20
days, by the end of this month, they will come very close together (2).
In fact on the first of December, the Moon also joins in and it will show its own distinct SHIFT in position relative to the two bright planets within a few hours after sunset on 1st December. On that day, at sunset (around 6:30 pm), the crescent moon will be BELOW the pair posing a beautiful show with Venus to the left and Jupiter to the right, slightly lower (3 and 5). But by 8.20 pm it will be IN LINE with the two bright planets (6). And by 9 pm while the trio are setting below the horizon in the west, the moon will be ABOVE two bright planets Venus and Jupiter (4 and 7).
What does it all mean? It shows that distant objects out there in space are moving about! The changes in positions relative to each other show that planets are object that move about – which we know they do because they circle the sun (in anticlockwise direction when looked down from above the north pole). In fact the word ‘planet’ in Greek means ‘wanderer’ which is thing that do not stay in one place. Ancient astronomers compared the positions of planets with that of nearby stars and found that separation between them changed significantly over time.
When they compared the positions of pairs of stars they found that stars do not shift about in the sky and maintain the same position all the time. Hence the shapes formed by connecting lines between stars came to be recognized as unchanging and the shapes (called constellations) are identified with familiar objects on earth such as animals (Leo the Lion), birds (Cygnus the bird), insects (Scorpius, scorpion) and humans (Orion the fighter god, Gemini the twins, Virgo the virgin, Sagittarius the archer etc.). Of course when you watch the sky for a few hours, or at the same time over several days, you will notice that the constellations are seen in different directions at different times of the night and different time of the year. But the shapes of the constellations remain the same. Hence we are sure that stars do not move about (which is strictly not true since they do move if seen over tens of thousands of year, which obviously you can’t see)
Hence any movement in the stars is associated with planets and moons. When planets circle the sun, their movement is seen in the sky as a movement from west to east. In the shifting show that we are witnessing at the moment, Venus is moving fast in its orbit around the sun, while Jupiter hardly moves because it is so far away from us. Since Venus is much closer to earth, its movement is very noticeable and it climbs up as expected, away from the west, to meet Jupiter by first December.
The change in the position of the moon on 1st December is also due to the movement of the moon towards the west. But since the moon is even closer to us than Venus, its movement in the sky is much more noticeable even over a couple of hours.
When looked at through a telescope, Jupiter’s four moons can be seen distinctly and appear as pinpoints, all in a straight line. The number of moons visible changes as they revolve around the planet. These two facts, the straight line alignment and the disappearance and appearance of the moons show that the moons are orbiting Jupiter in a flat plane. This is similar to how the planets themselves also orbit around sun, all circling in a wide flat plane.
Through a telescope, Venus shows up as a oblong shape in a telescope, since part of its face is on Venus’s night side. As the months go by Venus’s shape through a telescope will change dramatically as it approaches Earth in its orbit around the sun. Beginning with a circular disk when it is farthest away, its shape will change to half phase midway in its orbit. The half phase is seen because half of it is on the daylight side and the other half on the night side. This is similar to when the moon is in quarter phase, and for the same reason: being lighted by the sun one side which shows up as half moon.
From then on until the beginning of March next year Venus will become dramatic in its shape and size when viewed through a telescope. Its shape changes to a crescent shape while the size becomes three times larger as the planet makes its closest approaches to Earth by 10th March next year. Over this period, Venus’s brightness remains almost the same since the decrease in its lighted area visible to us is compensated by its increasing apparent size.
For more details of what is happening this month, visit our
website: www.astronomy2009.ac.tz for
a full article and a lot of useful information about astronomy in Tanzania
and on the International Year of Astronomy 2009 which is only a little more
than a month away.