Gravity and Tides in August Night Skies over Tanzania

Gravity and Tides in August Night Skies over Tanzania


Do you know why water in oceans and large lakes fills up and empties out regularly every twelve hours and even more every two weeks?  That is, do you know how tides arise? 

The most continual force that we are exposed to is gravity.  It keeps all matter in our Universe together.  Which means it keeps our world (Earth) together also - in a spherical shape on which we can live. Gravity is so invisible that we take it for granted.  Only when we have to jump that we realize that we are pulled back by this invisible “hand” that brings us down to earth, sometimes with a bang.  Gravity is always an attractive force between any two objects and it arises from the matter which we are all made up of.  The bigger the objects, the bigger the gravitational force. The closer the objects, the force becomes even larger and is increased by the square of the distance separating the objects.  Even between you and your friend there is an attraction but your bodies are two small for you to feel the pull of gravity between you two.  

Our Universe is made of massive objects.  One of them is our Earth, which is about 10,000 kilometers across.  The next massive body that is near enough to us is the Moon, which though it is relatively small, about 3,000 kilometers across, it is large enough to create a huge gravitational attraction.  It is this invisible force that keeps the Moon trapped continually to revolve all the time around Earth.  In fact the pull of the Earth is so large that it deforms the Moon by about half a meter along the line towards Earth. 

The Moon in turn also pulls the Earth and the force is strong enough to cause large bodies of water such as oceans and large lakes to be pulled towards the Moon. When the Moon is above the center of the ocean the water is pulled away from the shores of the oceans and we get an ebb tide where we see water pushed far away from the shore.  When the Moon is above the edge of the ocean, water is pulled in towards the shore and we get a high tide.  The pull of the Moon is also sufficiently large that the whole Earth is pulled slightly towards the Moon and so creates a bulge of water on the other side from where the Moon is.  Hence every day, as the earth rotates on its axis, we have two high tides and two ebb tides.

The next massive object after the Moon is the Sun.  This is so massive (one million Earths) that when the Moon and Sun are on the same side, the gravitational pull is even higher that normal and we get extremely large high tides or extremely low ebb tides.  Sun and Moon are in line with earth twice in a month, hence every month we get two extremely high high tides and two extremely low ebb tides.  So now we know how tides are formed.  The actual timing of the daily tides depends mainly on the time of the day that the Moon is above you.  However this is changed if the shoreline is not straight or uniform because presence of rocks and cliffs will turn the water away from the shore.  Hence the exact times of tides have to be calculated by experts dealing with tides.

Tidal forces affect all other objects in our Universe and shape their lives.  Gravity is the only force that acts over astronomical distances and gives birth to stars and makes stars to bunch up into galaxies and makes planets go around their stars.

Coming back to the night sky, Saturn and Mars form a close pair almost overhead towards the west, above Scorpio’s head.  They both shine with the same brightness but Mars is distinctly red compared to Saturn while both shine steadily without twinkling as planets usually do.  Saturn forms a beautiful sight with flat rings surrounding the planet.  It is well worth going out of your way to seek a telescope.  Mars does not show much features except that it will be seen as a small red spot through at telescope.  It is on its way away from Earth and so will keep getting dimmer. 

The morning sky continues to host the extremely bright planets Jupiter and Venus which are still close together just above the east horizon before dawn.  They are now separating after their closest approach on 18th August.  The morning star Venus continues to dip down the east horizon on its way to conjunction with the Sun after two months, after which it will enter the evening skies as an evening star.

The early evening sky is cool and crisp and hence very steady for good viewing. Among the prominent constellations are the long, winding Scorpio, and Sagittarius the archer.  The Southern Cross lying on its side close to the southwestern horizon points south.  In the north you can make out Cygnus the swan as a cross with the bright Deneb (A) at its tip.  Among the bright stars you can try some with famous names.  These are, from west to east, Spica (B) in constellation Virgo, Arcturus (C), Alpha (D) and Beta Centauri (E) the two bright stars in the southern skies that point to the Southern Cross, the red star Antares (F) in the neck of Scorpio, Vega (G), Altair (H) and Deneb (A) all in the north and Fomalhaut (J) just rising in the east at 8 p.m

The International Space Station (ISS) will be seen on Sunday August 31, rising from north at 6:54 pm and will move close to the east horizon and will disappear suddenly in the east at 6:59 pm. On September 1 it will move similarly but in the west horizon, rising at 7:41 pm and will disappear suddenly at 7:46 pm when it is in the south west. The ISS will be provide a clear view on September 2 from 6:53 pm from the northwest. This extremely bright satellite will cut through the middle of the sky over nine minutes and will disappear in the south east horizon. On the evening of August 4, it will again hug the west horizon sky, rising in the northwest at 6:54 pm and will sets in the south at 6:59 pm.  For exact timings for your own locations, visit and enter your position.