August Night Skies Over Tanzania - Pluto and Persides

Pluto has recently been in the news following remarkable discoveries by the New Horizons spacecraft launched in January 2006, a few months before it was declared a dwarf planet by the International Astronomical Union. The spacecraft flew by Pluto last month and sent back the first extremely detailed pictures and measurements ever made. Pluto, the farthest former planet, is on average, about six billion kilometers away, 40 times more than our distance to the Sun so, its light and radio signals take five hours to reach us. Remember that light goes 300,000 km in one second, which gives you some way of imagining how far Pluto is. The dwarf planet takes 250 years to go around the Sun so it has only traveled only a third of its orbit since its discovery by Tombaugh in 1930.

All the planets had been studied from close up during the Voyager 1 and 2 missions launched in 1970s. However they could not include Pluto in these flyby missions due to its position. New Horizons was specifically launched to fulfil the aim of studying all the planets at close range. After nearly ten years of travel, with gravitational catapult assist from Jupiter, New Horizons approached and flew by Pluto on 14 July just 12,000 kilometers away. Such a close successful flyby is equivalent to firing a gun so that you miss an object placed at the other end of a football ground, by just one tenth of a millimeter. The spacecraft will now fly on, sending back more images and measurements it has collected from Pluto and go on deeper into the solar system to flyby other distant objects and eventually out of the Solar System. Incidentally, the two Voyager space crafts have now crossed out of the Solar System and are in interstellar space, still regularly sending information back to Earth.

New Horizons found that Pluto has more moons than those around the inner planets, and has a million times the amount of atmosphere that Mercury has. Its surface has distinct features with water-ice mountains bigger than many large mountain ranges here on Earth. Its frozen plains are surrounded with ridges that suggest there is geological activity that causes changes to its surface. This extreme activity on Pluto has now given support to those who were in disagreement about the decision to make Pluto a dwarf planet. New Horizons came just in time to provide detailed information that Pluto is not a quiet dwarf planet so this debate will continue in the years to come.

The early days this month, from 10 to 15 August are prime times to observe the Perseid meteor shower which produces around 20 meteors per hour at our latitudes. When the Earth moves in its orbit around the Sun, it is rushing through space at a speed of more than 100,000 kilometers per hour. When the orbit of the Earth crosses the orbit of past comets it strikes concentrations of dust particles left behind by the comets. During the Perseid shower which peaks on the night of Wednesday August 12, Earth will hit material left behind by comet Swift-Tuttle whose dusty-ice ball is softened and releases material during its close approach to the Sun (and Earth) in elongated orbits every 133 years, the last of which was in 1992. When such dust particle strike the atmosphere they are heated by friction as it speeds through the air and glows brightly, showing up as lines in the sky which we call meteors or shooting stars since it looks as though a star has fallen from the sky.

The Perseid meteors are often persistent and visible for longer time even after the meteor has passed. This light is produced by ionization of the air through which the meteor has passed.

Maximum meteor shower occurs when the Earth hits the comet dust head on, which is at sunrise and in overhead sky, hence the best time to see the most meteors is when it is still dark just before dawn. Earth will be in the most concentrated dust region on the night of 12 dawn of 13 August and at that time the Moon will be in a waning (decreasing) crescent phase so there will be minimum disturbance from moonlight. The following day, August 14 is a New Moon so there will be no moonlight so it is also worth going out to watch the meteors.

From around 10 pm on wards there can be a few earth grazing meteors that strike the atmosphere not head-on, but at a glancing angle. These produce long slow trails that pass across the sky overhead.

The position in the sky from which the dust strikes the atmosphere is called the radiant, and meteors trail lines when extended back, will meet at the radiant which shows that the meteors are radiating or originating from space around that position. The radiant of the Perseid meteors is in the Perseus constellation from which the shower takes its name. The Perseus constellation is towards the north and does not rise more than 30 degrees at maximum hence northern hemisphere observers will enjoy this shower even more.

At midnight, the radiant is at the horizon, hence before and around midnight, grazing meteors will appear to originate from the horizon and streak upto the overhead skies. By dawn when maximum meteors appear, the radiant is overhead with meteors shooting out in all directions from a point overhead.

The best way to observe meteors on the night of 12/13 or 13/14 August is to pick a dark place where the north east horizon is not obstructed. Lay on an armchair or on a mat on the ground and scan the whole sky. Meteors appear without any particular pattern so you have to be patient and keep your eyes relaxed and observant for long periods. The satisfaction of seeing a steady streaks of light is immense since most people will have seen just one or two chance meteors only. If you can see a bolide, which is an even more dramatic event when a larger particle breaks up while you are watching it, your day will definitely be brighter since you would be among the extremely few people who have seen such an event.

Venus and Jupiter have bid us farewell at the end of last month. Jupiter will be seen again in the early evening skies after about six months, while Venus, the evening star that was with us for the past eight months, will be hidden in the Sun’s glare for a couple of weeks, after which it will reappear in the dawn skies as a morning star at the end of this month.

Saturn is the only visible planet in the evening skies and shines almost overhead to mid skies close to the mouth of Scorpio. Mercury makes a highest entrance in the western skies this month, rising to more than 20 degrees above the horizon at sunset by the end of August, and hence provides a very good opportunity to see this elusive planet. Visit www.astronomyintanzania.or.tz for updates and further information.

Among the evening constellations are the long, winding Scorpio, and Sagittarius the archer. The Southern Cross lying on its side close to the southwestern horizon points south. In the north you can make out Cygnus the swan as a cross with the bright Deneb (A) at its tip. Among the bright stars you can try some with famous names. These are, from west to east, Spica (B) in constellation Virgo, Arcturus (C), Alpha (D) and Beta Centauri (E) the two bright stars in the southern skies that point to the Southern Cross, the red star Antares (F) in the neck of Scorpio, Vega (G), Altair (H) and Deneb (A) all in the north and Fomalhaut (J) just rising in the east at 8 p.m

The best view of the International Space station will be on 31 August, when it will cross the sky starting from northwest horizon at 6:38pm and reaches nearly 70 degrees at 6:41pm shining extremely brightly. Just before this, at 6:40pm it will pass close to Saturn in the sky. It will then drop towards the horizon and disappear after 6:44pm.