Exploring the Equinoxes and September Night Skies over Tanzania

We are once again upon the equinox with 23rd September as the day of Equinox. The length of day and night will be exactly equal (12 hours) throughout the world. The Sun will be exactly overhead at midday for places on the equator.

But we in Tanzania are not at the equator so you may ask, is the Sun overhead at our place at midday? The first thing to note is that the Sun is overhead at the equator twice in a year, during the March Equinox and during the September Equinox that we are experiencing now. This happens due to the inclination of the Earth’s axis to the plane of the Earth’s orbit around the Sun. You can visualise a flat table as the surface on which are the Earth, the planets and the Sun, with all the planets as ball of different sizes rotating around the stationary Sun. Now imagine that you are spinning or twirling the Earth ball in your fingers but you would not keep your fingers upright while spinning the ball. You would have to slant your fingers away from the vertical by 23 and a half degrees. This inclination of the Earth makes the top (north) side of the Earth lean towards the Sun.

With the same slanted fingers twirling the Earth ball, if you move it along its orbit to position it diametrically opposite the first place you can imagine that the bottom (south) side would be leaning towards the Sun. The side that leans towards the Sun will be hotter because the Sun’s rays will fall more vertically there. The other effect is that while you move the Earth with your slanted fingers along the orbit you will notice that half way between the diametrically opposite positions, neither the north side nor the south side face the Sun and in fact the Sun is facing the equator. This situation is called Equinox.

Hence during the six months that it takes for the Earth to revolve half way around the Sun it will face its north side towards the Sun, then the equator will face the Sun (at equinox) and after that the south side will face the Sun. Completing the remaining half of the revolution the Earth’s equator will once again face the Sun (equinox again) midway to its original position. Hence this explains why there are two equinoxes.

When this situation is now viewed from inside the Earth, we feel that the Sun is moving instead of the Earth, just as we feel that the Sun is moving during the day while actually it is the Earth is the one that is revolving. Viewing the Sun from inside the Earth over the course of the year for example at sunrise or at sunset we will see the Sun’s position at sunrise or sunset changing day by day. At Equinox the Sun will rise exactly due east and sets exactly due west.

At maximum the Sun’s position at sunrise or sunset will shift by 23 and a half degrees, being that much angle north in June and the same angle south in December. Hence during the year the Sun shifts position 23 and a half degrees north and south of the east position. At midday the Sun will be exactly overhead at the equator during the two equinoxes, and during the year it will shift by upto 23 and a half degrees away from zenith toward north or south.

During this shift, if your position on Earth is within the tropical latitudes of 23 and a half degrees north and south of the equator, you will also find that the Sun is exactly overhead twice a year at your latitude. For us in Tanzania who are between 2 and 11 degrees south of the equator the Sun will be overhead between February 23 and March 18 before the March equinox and between September 26 and October 22 after the September equinox.

It is interesting to look out for the exactly overhead sun during this period because shadows of all vertical objects disappear below the object and you cannot see them. If you stand in the midday Sun on these two days you will not be able to see your own shadow. Go out and find out which day during the September 26 and October 22 period and find out on which day your shadow disappears at midday. Learn more about the zero shadow day from http://planetcalc.com/1875/ and http://www.iiap.res.in/iya09/VPwkshop/talks/RugvedPund.pdf

The planet Mercury, Mars and Saturn are well positioned in the western sky. View them after sunset as soon as darkness brings them out in the sky. The picture shows a simulation using Stellarium of their positions in the sky about one hour after sunset on Sunday 28 September 2014.

Watch out for the red Mars-Antares pair’s positions shift day after day as Mars moves in its orbit. You will be witnessing the movement of a planet in space. Marvel at such an opportunity. Antares is a red giant star while Mars has red soil so both look distinctly red. Mercury is difficult to look for but through a telescope you will see it in half phase, another marvel to witness. Saturn presents the most beautiful sight of all through at telescope with its brilliant flat ring system. The Moon will be a thin crescent completing a beautiful view of the celestial objects in the sky.

You can follow the progress of the zero shadow day for various towns and cities in Tanzania as well as the movement of the Mar-Antares pair on www.astronomyintanzania.or.tz

The Milky Way stretches as a band of numerous stars across the middle of the evening sky, passing through the Southern Cross in the southwest, through Sagittarius overhead, to Cygnus in the northeast. The portion of the Milky Way close to Sagittarius appears as a cloud that is not a real cloud but nebulae of dense interstellar matter that is hiding from our view a powerhouse that is a supermassive blackhole at the center of the Milky Way galaxy.

The Scorpio constellation can be clearly identified by its namesake the scorpion and can be seen high in the western sky. The eastern sky has two “birds”; one to the northeast, where you will see the Cygnus with its body and wings making a wide cross, while in the southeast you will see the smaller bird Grus with its head twisted sideways. Try to become familiar with the brightest stars by their names and relative locations. Enjoying the sky will not be as overwhelming as it might seem