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.
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