September Night Skies over Tanzania

By Dr. N. T. Jiwaji

Email: ntjiwaji@yahoo.com

September is a month of balance for the world.  The most differentiating aspect around the world, astronomically speaking, is the length of a day in different parts of the world.  While in the North and South polar regions, length of day can be zero (i.e. Sun does not rise) during  half the year, during the other half, the Sun does not set so there are never ending days.  Even in equatorial regions, lengths of days can be noticeably different during cool and warm months.

However, on Monday 23rd September, we will witness one of the two times during the year that length of day and night is exactly equal.  This is the day of the equinox.  The September equinox is called the southern equinox, to differentiate it from the March equinox, also called the vernal equinox or the northern equinox.

During the equinoxes, as will happen this Monday 23rd, the earth's axis of rotation (the line connecting North and South poles) will be exactly perpendicular (i.e. 90 degrees) with the line between the centers of Earth and Sun.  The exactly equal day and night occurs twice during in a year because the axis of the earth is tilted by 23 degrees angle with the plane of revolution of the Earth around the Sun.  Hence most of the year, except on those two equinox days, parts of the earth will be either pointing towards the Sun, or in the opposite hemisphere, pointing away from the Sun.  Only on the two equinox days, day and night are exactly 12 hours each.

Another interesting effect of the tilt of our Earth is that the Sun is never overhead during most of the year.  During equinoxes, the Sun is directly overhead only at the equator at local noon.  All places that lie within the tropics, between the Tropic of Cancer (latitude 23.5 degrees north) and the Tropic of Capricorn (latitude 23.5 degrees south), experience overhead Sun twice in a year.  No place on Earth north of the Tropic of Cancer or south of the Tropic of Capricorn can have overhead Sun.

For us in Tanzania, we experience overhead Sun soon after the September equinox or just before the March equinox.  Hence be prepared for overhead sun between 30th September and 20th October, depending on whether you are in the north or south of Tanzania.  For example in Dar es Salaam, overhead Sun will be on October 11 at local noon at 12:10 pm.  So go out that day into the midday sun at local noon and watch your shadow disappear.

In the cool clear nights of September, brilliant Venus continues to glaze low in the western skies, from sunset onwards up to nearly 9 pm.  Saturn is close by, forming an attractive pair that separates day by day as Venus rises higher while Saturn shifts lower and lower, eventually disappearing from the evening skies when it goes below the western horizon by mid-October.  Jupiter is an early morning sky shining high above the eastern horizon.

The Milky Way stretches across the middle of the evening sky, as a bright band of myriads of stars mixed with wispy cloudy patches of dense collection of stars.  The milky band passes through the Southern Cross in the southwest, through overhead Sagittarius to Cygnus in the northeast. Close to Sagittarius is a dense collection of interstellar matter in the direction of the center of our Milky Way galaxy which is believed to be a super massive black hole.

The Scorpio constellation can be seen high in the western sky and is clearly identifiable because of its distinct shape of a scorpion. Cygnus (the bird) constellation can be identified by the cross made by its long body and wide wings. The square of Pegasus is also clearly identifiable as a big square marked S in the sky map.

Eight of the top twenty brightest stars in the sky are visible at this time in our evening skies. The stars marked A to H are listed here with their names and its rank in brightness. A – Formalhaut (18th), B – Altair (12th), C – Deneb (19th), D – Vega (5th), E – Antares (16th), F – Alpha Centauri (4th), G – Arcturus (3th), H – Spica (15th).  Enjoy the bright  stars in clear skies  during these September nights.