Looking for the Idd Crescent and a Comet in July Night Skies over Tanzania

Looking for the Idd Crescent and a Comet in July Night Skies over Tanzania

Looking for the Idd Crescent and a Comet in July Night Skies over Tanzania




The Moon is the most prominent astronomical object in the night sky, so much so that we often don’t notice how it is changing day by day.  For half the month you will not even see it unless you are a late sleeper or very early riser. Once a month it enters the night sky as a beautifully crafted crescent low near the horizon in the western skies in the evening just after sunset. Over the next 14 days it shifts by about 10 degrees every night, growing in size and brightening until it appears on the opposite side of the sky.  It rises majestically over the eastern horizon as a bright Full Moon while the Sun sets in the west.


The Moon has been used to mark the lunar month for many societies around the world, starting from Babylonians.  Muslims, for example observe the First Crescent Moon to start and end each lunar month. Being an astronomical object, and using the developments in science and technology available today, the Moon has been studied by many scholars to determine its position, size and shape to assist in the observation of the First Crescent.  This is the first visible crescent after the New Moon.  The New Moon marks its longitudinal alignment with the Sun, hence impossible to see.


A First Crescent is a small thin crescent, close to the bright western horizon sky at sunset.  Ii is not possible to contrast the dim brightness of the First Crescent Moon against the bright sunset sky unless the crescent is sufficiently progressed in its orbit and makes a sufficient angle in the sky above the setting Sun and above the horizon.  This limiting angle, known as the Danjon limit, was studied by the French Astrophysicist of that name, who first used scientific method in 1932 to measure the astronomical limit of visibility of the First Crescent. Many other astronomers have followed up measurements of these observations.  The currently accepted limit of visibility of the First Crescent varies between estimates of more than 5 degrees to 7 degrees away from the Sun at sunset. Even at these limits it is extremely difficult to observe unless the position its expected position is used to focus the search.  It is extremely dangerous to use a telescope under these conditions to observe since any part of the Sun if above the horizon can instantly damage the eyes.


The next New Moon will occur on Sunday 27th July at 01:42 Hrs after midnight.  By time it is Sunset of that day, it will have progressed by only 6 degrees away from the Sun at sunset.  Though it is just within the visibility limit it will allow only a five minute window of fleeting observation by the time it drops below its 5 degree visibility limit and a few more minutes before it sets. None of the landmasses of our world to our west (e.g. The Americas) will be sufficiently far away for the Moon to progress significantly beyond the Danjon limit within the day of 27th July.


By sunset of the next day, Monday 28th, the crescent will be far higher and sufficiently away from the Sun to become clearly visible in the western sky just after sunset, and will provide nearly an hour of observation time before it sets below the horizon.


Another interesting astronomical object that has recently appeared in the night sky is the appearance of the comet “Jacques” in the night sky just before sunrise. It has reappeared after rounding the Sun in its elongated orbit from depths of the outer solar system. It is at magnitude +6 which is just visible to naked eyes in dark skies but it is best to scan for it using a pair of binoculars or a even small telescope.  It can be seen before sunrise and just before the Sun rises, it will be in about 25 degrees above the horizon and about 10 degrees above the extremely bright Venus and close to a bright star forming one of the horns of the Taurus (Bull) Constellation.


As days go by, though bright Venus will slide lower towards the eastern horizon, the comet will continue to rise into the darker parts of the dawn sky so should be easier to locate.  Its closest approach to Earth occurs on August 29 when it will be about 85 million kilometers from us.  We have more than enough time to plan an early wake day and observe this rare sight.  Use the opportunity of the Perseid meteor shower on 12-13 August to kill two birds with one stone.  At this time, the comet will be quite high in the sky in the Perseus constellation though the bright Moon can reduce naked eye viewing. We will be following the progress of comet Jacques through our Astronomy In Tanzania website: http://www.astronomyintanzania.or.tz


Yet another astronomical object worth following in the early evening skies is the red planet Mars.  Since it is close to Earth, its movement in its orbit shows up as a significant movement in the sky across the background stars.  The word “planets” is derived from the Greek word “wanderers” as planets were observed to wander, i.e. move across the background of stationary stars. At the moment Mars is close to a bright star Spica in the Virgo constellation. 


At the moment, Mars and Spica are separated by about one degree, which is the amount of sky that would be covered up if you held your index finger at an arm-length.  As days go by you will be able to see a clear separation of these two stars and you can estimate that Mars and Spica will be separated far more than your finger-width.  By the end of the month the pair will be more than five finger-widths apart. By the end of next month Mars will be close to Saturn which does not move significantly since it is much farther away from us.


Saturn is close to overhead zenith skies near Mars and near the three stars forming the mouth of Scorpio constellation.  It shines with a sharp brightness and can be distinguished from nearby stars by comparing its steady light with that of the twinkling of stars.  Through at telescope, Saturn is one of the most beautiful objects you can see in the Universe.



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) will provide a satellite show on 31 July and 1 August when it will rise from southwest horizon and the football sized space station will glide as a bright star across the sky towards northeast over six minutes.  On 31 July it will start its flyby at 7:34pm and will keep to the western sky rising to a maximum of 45 degrees, while on 1 August it will appear from 6:45pm and will glide in the eastern sky rising to 65 degrees above the horizon.  More accurate times and directions can be found at http://heavens-above.com by inputting your location coordinates.