May Night Skies over Tanzania

By Dr N T Jiwaji

ntjiwaji@yahoo.com

A last-view alignment of three planets awaits us this end of month when giant Jupiter, brilliant Venus and ephemeral Mercury come together to present a visionary delight.  On Saturday 25, the three planets will form an upright right angled triangle about 15 degrees above the western horizon at sunset. Over the next week the wandering planets will shift positions day by day, as Jupiter slowly sinks away from view setting in the western horizon, while Venus and Mercury rise in the sky, coming further and further into the evening view.  By the end of the month, the three planets form a clear straight line in the sky with Mercury highest, Venus in the middle and Jupiter below. With a clear, cloud-free view of the west, you should be able to see the triplet until about 6:45 pm.

This is your opportunity to see actual movement of celestial objects.  Stars in the sky are a confusing jumble to many people.  Stars are extremely distant bodies so they hardly change positions in the sky.  Planets, on the other hand are relatively close to us so any movement in their orbits shows up by changing positions relative to the stars.  During the coming week, since three planets are all moving in their own orbits, their motion is far easily seen.  The month of May is at the extreme end of our rainy season; hence the skies should open up sufficiently to allow you to catch a glimpse of the planetary triple treat during the coming week.

While Jupiter continues to dip below the western horizon as the days go by, Venus and Mercury continue to climb higher and higher in the sky.  In fact by mid-June, Mercury will rise extremely high in the sky, to nearly 25 degrees above the horizon at sunset.  This is a near record elevation for Mercury since it orbits closest to the Sun and so remains regularly close to it.  By following Mercury in the evening skies during the next three weeks, you will be among the exceptional few who will have seen this fast moving planet.  

Venus will continue to become extremely bright as it rises higher in the sky over the coming months, leaving Mercury well behind by the end of June.  It will remain brightly visible as a beacon in the western sky until the end of the year.

Another planet that is in our evening skies is Saturn, which shines with a sharp steady brightness in the eastern sky.  As the night progresses, it remains visible until close to dawn as its position shifts from east to west due to rotation of our Earth.

Besides naked eye viewing of visible planets, you can enjoy their views through a telescope.  Venus and Mercury appear as orange dots showing that they are indeed spheres in the sky, but you they are mostly featureless in ordinary telescopes.  After mid-June, Mercury becomes crescent shaped when the planets dark side begins to face us during its inner orbit around the Sun.  Venus also takes a similar shape later in the year, after mid-November when its dark side begins to face us since its orbit is also inside that of our Earth.

The true joy of a telescope comes when viewing Jupiter and Saturn which show distinct features of the planet itself.  Jupiter’s equatorial clouds become visible as two parallel bands even through at modest telescope.  The giant planet has four moons that are easily visible as tiny stars when viewed through a telescope.  They appear along a straight line through the planet’s equator, changing positions hour by hour as they orbit the planet.  Saturn provides the most beautiful view through the telescope, with a vivid flat ring surrounding the bright central sphere.  Its angle of view from Earth is sufficiently inclined to show it as viewed from above at a slant angle to give a truly memorable view.

The most recognizable constellation, Orion, bids us farewell and sets by 8 p.m. Leo dominates the skies, though the full glory of this constellation with an inverted question mark forming the head of the lion, and its lounging body, can easily be obscured by city lights.

The Pointers, the Southern Cross in the southern sky and the Big Dipper in the northern sky, are well positioned from early evening and are high in the sky so they can be used to locate south and north (see map). Scorpio pops its front claws early in the evening and by 10 p.m. the sweeping, curving tail with the sting at its end formed by a close pair of stars is easily made out.

From our unique geographic position close to the equator, we can see 16 of the 21 brightest stars in the evening sky between 7 and 8 p.m. They are (in order of brightness, with their constellations in brackets) SIRIUS (the Dog), CANOPUS (Carina), ALPHA CENTAURI (Crux), ARCTURUS (Bootes), CAPELLA (in Auriga, just off the map, and sets at 7:30 in southeast), RIGEL (Orion), PROCYON (the Dog), BETELGEUSE (Orion), BETA CENTAURI (Crux), ALDEBARAN (in Taurus, just off the map, sets in the west at 7 p.m.), ACRUX (Southern Cross in Crux), ANTARES (Scorpius), SPICA (Virgo), POLLUX (Gemini), MIMOSA (Crux), and REGULUS (Leo). Identify the brightest stars using the map and you will understand better how to relate the sky to the star map, and the distortions of distances, especially close to the horizon.

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