Idd Crescent and July Night Skies over Tanzania
This month once again thousands of eyes will be turned upwards in the west horizon looking for a sacred celestial body. The Moon is the closest celestial body and continually in our eyes at night shifting position and changing shape as it orbits the Earth. The sight of one particular shape, the First Crescent marks the beginning of the month in many cultures that use the lunar calendar. Muslims will be awaiting the First Crescent to mark the end of Ramdhan fasting and celebrate Idd the following day. This month the First Crescent is expected to occur on either Thursday 17th or Friday 18th July. There is uncertainty in our ability to see the First Crescent because it is a very thin crescent that appears very close to the west horizon within the glowing atmosphere around the just set Sun.
The First Crescent is marked just after the New Moon, when the Moon has moved away from its alignment with the Sun. At New Moon, the Sun is behind the Moon, hence it is impossible to see the Moon because its night side is facing us. Immediately after that, since the Moon moves in its orbit around Earth, the Moon will no longer be in front of the Sun and will have moved slightly eastward, away from the Sun, as viewed from Earth. Once this happens, a small part of the daytime side of the Moon will come into view and forms a very thin crescent very close to the Sun.
When this crescent is visible from Earth, it is termed the First Crescent which will be thin and close to the horizon so will set within a short time. If the crescent is too thin it will be extremely difficult to see in the bright glow of the atmosphere around the setting Sun and will set before the sky has become dark enough.
The key factor for the First Crescent to be able to be seen is that the Moon should get enough time to move in its orbit after the New Moon so that is it sufficiently away from the Sun, and the crescent is large enough and high enough, for the sky to become dark and people have enough time to view it in the west horizon before the crescents gets too low. Since the crescent can only grow with time, once the First Crescent is detectable from one place on Earth, all other places in the world that are west of that location will reach nightfall later hence all other places to the west of the first observed location will also be able to view the First Crescent.
The New Moon this July is on Thursday 16th at 4 am in the morning Tanzania time. So by sunset on that day, 16th, the Moon will have had only about half a day to move in its orbit, which is not sufficient to move it away from the Sun. Hence at sunset on 16th, the crescent will be only 0.4% in size and only 5 degrees above the west horizon, which is practically impossible to see with naked eyes in the bright sunset sky. However, by the next day’s sunset on Friday 17 July, the First Crescent will be 2.4% in size and 16 degrees above the horizon and will set around 7.30 pm. This will allow sufficient time the sky to darken and for people to observe the First Crescent on Friday 17 July and we can expect the first day of Idd to be celebrated on Saturday 18th.
The brilliant Venus-Jupiter pair has changed places after coming closest at the beginning of the month. Venus shines ever brighter now as it moves closer to Earth in its orbit around the Sun. Through even a telescope or binoculars, Venus is seen as a thin crescent growing thinner by he day. However since the planet is getting closer to us, its angular size is getting bigger and bigger making it shine brightly in spite of its crescent getting thinner.
Venus and Jupiter are now further apart again and the crescent Moon will join the pair on 18 July forming a very attractive shape of an equilateral triangle. A bright star close to the Venus-Jupiter pair is Regulus, the brightest star of Leo constellation, but it is much fainter than the two planets. The planet pair will shift slowly downward towards the west horizon day by day and will disappear by mid next month.
Mercury enters the evening sky towards the end of this month. It is the closest planet to the Sun hence is always never far from the Sun and keeps close to the horizon. By 31 July it will be about 8 degrees above the west horizon at sunset hence you can set a challenge to detect a faint point in the bright horizon sky. By 7:45 it can be dark enough to be detected so the challenge can be who detects it first on the days following July 31. It will continue to rise in August, reaching a very high altitude of 24 degrees above the horizon at sunset and by that time it can definitely be seen. On 7 August Mercury will be seen close to Jupiter hence the bright planet will guide you to locate Mercury. Take time to see this mercurial planet since we are lucky to be in a location where it can be followed with the naked eyes.
Saturn keeps close to the tentacles of Scorpio and climbs high in the sky and can be seen almost over head at 8 pm. For further details of Tanzanian night skies see http://www.astronomyintanzania.or.tz
The Asteroid Day that was marked on 30 June is a new initiative to highlight the dangers of our planet being struck by a huge body with the potential of destroying large populations of life on Earth. There are a million of asteroids with the potential to destroy but only 10,000 have been discovered and are being tracked to alert us of their dangers. Hence there is a 100 times more effort that has to be put in to detent and monitor Near Earth Asteroids (NEO). Hence governments of the world need to be aware of this danger and urges people to sign a petition to support this action. Visit http://www.asteroidday.org/declaration for more information and support this movement.
The southern skies are filled with very bright stars that would be hard to hide! The north and south direction pointers, that is the Big Dipper and the Southern Cross, are still high enough in the evening skies and can be used to mark the north-south direction very well. Scorpius in the dominant constellation this month, occupying the overhead evening sky with its three stars forming its tentacles, the red star Antares in its neck and a long winding tail that ends in a close pair forming the sting. Scorpius is the only constellation that does full justice to its namesake, the scorpion and is unmistakable to even a casual stargazer. Below Scorpius, try to identify Sagittarius (the archer). This constellation marks the direction of the center of our Milky Way galaxy and you will notice dense concentration of stars here. The band marking the Milky Way contains numerous stars and dust patches and stretches from the southwest, passing through the Southern Cross and Sagittarius and upto Cygnus (the swan) in the northeast. Leo (the lion) with its distinctive inverted question mark head is low in the western horizon and will be lost after this month.
Among the brightest stars noticeable in the July skies are: Alpha and Beta Centauri in the south form the pair that points continuously towards the Southern Cross; the fourth brightest star Arcturus is overhead towards the north and the fifth brightest star Vega rises in the northeast. Other bright stars you will easily notice are Altair which rises in the east and Spica, the brightest star in the Virgo constellation can be seen almost overhead towards the west.
The International Space Station (ISS) is set to make a majestic crossing across the Tanzanian evening skies between 28th and 30th July, with the southern areas on 28th, coastal central areas seeing it on 29th, and western and northern parts seeing it on 30th July. Look up the exact times for your location using the websites http://www.heavens-above.com and http://spotthestation.nasa.gov