April 2011 Night Skies over Tanzania

April 2011 Night Skies over Tanzania

The sky this month will be more frequently hidden from our view due to the rainy season. On the positive side, when the clouds clear, we will see a much darker sky, allowing a crisper viewing of many more stars. The dust collected in the air during the dry hot season has been washed away by the rains and will not scatter back (or reflect) light from the ground and reduce light pollution. The cooler temperatures will cause fewer ripples of the air making the stars crisper due to less twinkling.

This month we scintillating in the early night skies, eight of the top ten stars in the sky. Locate and identify these stars using the sky map: 1. Sirius; 2. Canopus; 3. Alpha Centauri; 4. Aructurus; 5. Vega (not seen), 6. Capella; 7. Rigel; 8. Procyon, 9. Achernar (not seen); and 10. Betelguese. We are lucky indeed, because we are in the Southern hemisphere, where the sky has the densest collection of stars.

The brightness of stars is measured in “magnitudes” where the brighter the star, the lower the magnitude number, and vice versa. Faintest stars that are visible to naked eyes have a brightness of sixth magnitude (i.e. magnitude 6). For brighter stars, the magnitude is lower than six. Hence the brightest stars have negative magnitudes!. The brightest star in the sky, Sirius, has magnitude -1.5, while the tenth brightest star is magnitude 0.5. Among planets, Venus is extremely bright, with magnitude -4, while Jupiter is also very bright with magnitude -3. Saturn, which is currently visible in our night skies, has magnitude 1, which is not very bright. Our Sun has magnitude -26 while the full Moon has magnitude -12. Remember that -12 is a bigger number, so represents a lower brightness, than -26. Stars that are not visible to the naked eyes have a magnitude numbers higher than 6. The faintest star seen (through a telescope) is magnitude 28.

Remember that we have described above the brightness of the stars as we see them in the night sky. However, a very bright star that is very far away will look less bright when compared with a fainter star that is very close to us. Therefore, the brightness magnitudes mentioned above are called “apparent magnitudes”.

To describe the actual brightness of stars we use another scale called “absolute magnitudes” in which the brightness of stars is compared when stars are placed (mathematically) at a standard distance from Earth. On this scale, our Sun gets an absolute magnitude of 4.8 which is not very bright and would be just visible with naked eyes.

Having just passed opposition Saturn is visible in the east after sunset and remains in the sky throughout the night, setting just before sunrise. Though not very bright, Saturn has a piercing brightness that allows you to identify it among the faint stars nearby. This is possible because light from a planet does not twinkle, while true stars twinkle.

Through a small telescope, Saturn presents a wonderful sight with a broad rings system that makes it look like a head with a hat. A more powerful telescope would reveal gaps in the rings and would show bands on the planet itself, together with a few of its more than 60 moons. The ring itself is made up of millions tiny and not so tiny pieces orbiting close enough to be seen as a broad ring when viewed from a distance. Saturn lies in the Virgo constellation.

Prominent constellations that can be seen are Orion, Taurus, Gemini, and Sirius, all currently crowded in the western sky. The Milky Way, which contains these constellations, stretches from northwest to southeast.

The constellation that climbs high in the eastern skies is Leo. This has a distinctive curve of stars that looks like a backward question mark makes the head of a lion. The bright star Regulus, at the bottom of Leo’s head is a triple star, with two of them easily separated through a pair of binoculars.

The direction pointers, the Southern Cross and the Big Dipper (shaped like an inverted saucepan) are high in the sky and are visible throughout the night. The long diagonal of the Southern Cross points towards the south while the edge of the pan in the Big Dipper points north.

- End -