The Night Sky this Month

August 2010 by Russell Eberst

Now that dark nights have returned, observers can take advantage of the increasing length of the darker hours to recognise some of the constellations of Summer and Autumn. The Summer triangle consisting of the bright stars Vega, Deneb and Altair remains prominent in the evening sky, and will continue to do so for a few months yet. But already becoming better placed are the stars and constellations of the Autumn.

These include Pegasus, Andromeda, Cassiopeia, Cepheus and Perseus. Once familiar with these bigger, more obvious groups, the observer may like to identify some of the smaller and perhaps less known constellations such as Sagitta (the arrow), Vulpecula (the fox), Equuleus (the little horse) and Delphinus (the dolphin).

Well to the south of these can be found the zodiacal constellations of Capricornus, Aquarius and Pisces. These presently host the three large gas giant planets Jupiter, Uranus and Neptune.

Neptune is now near the borders of Capricornus and Aquarius, very close to its discovery position. It has now completed one orbit of the Sun since it was found by Galle and d'Arrest on September 23, 1846. Taking 164 years to complete one revolution, Neptune represents the last of the classical planets. Before 2006, Pluto was included among the planets, but has now been downgraded to the status of 'dwarf planet'.

Uranus is in Pisces, slowly retrograding (moving westwards). At magnitude +6, Uranus is readily visible in binoculars. It can be spotted 2-3 degrees to the west of the brilliant planet Jupiter, this separation decreasing to 1.8° by the end of August.

Jupiter rises in the east before midnight and reaches an elevation around 34° in the south in the dawn twilight. Incidentally it proves to be a useful guide to finding the International Space Station in the pre-dawn sky for observers in southern Scotland. Predicted times of passes can be found on the Heavens-Above.com website.

August sees the evening grouping of three planets, Venus, Mars and Saturn. By far the brightest of these three is Venus, and it is therefore the best guide to locating the trio. Venus is also the nearest of the three, and shines at magnitude -4.4. It reaches its greatest eastern elongation on August 20, when it has its widest separation from the Sun in the sky (46°).

However, for observers in high northern latitudes, the low angle that the ecliptic makes to the western horizon at this time of year means that Venus will be rather poorly situated.

In the southern hemisphere, or near the equator (such as the Seychelles) the position is much more advantageous, with Venus being seen against a dark sky and setting more than three hours after the Sun. On August 20 (the date of the elongation), Venus will be passing Mars, which is very much fainter. Its magnitude is +1.5, or nearly 250 times fainter than Venus. They are separated by under 2°. Mars is now on the far side of the Sun, and therefore its disc is tiny, and unlikely to reveal any detail to Earth based observers.

At the beginning of the month, Mars and Saturn are in close conjunction, about 2° apart with Saturn the more northerly. By August 8, Venus sweeps past Saturn, with Venus about 2°.7 to the south of Saturn. Further additions to this gathering are the crescent Moon on August 12, with Mercury and Vesta (minor planet) also in the vicinity.

This tight configuration should provide an excellent opportunity for those wishing to photograph some of our near solar system neighbours. The Moon will be just past Full when it passes Jupiter (and Uranus) on the morning of August 27, but is about 6° to the north of this pair of planets.

The warm nights of August can encourage observers to remain outside for prolonged periods, watching the sky for hours on end. It is fortunate that one of the best meteor showers of the whole year occurs around August 12.

A regular feature of our August skies is the meteor shower known as the Perseids. It is so called because the meteors belonging to this shower appear to radiate from a point in the northern section of Perseus. For this shower, the comet responsible is Comet Kegler first observed in 1737 and again on its next return in 1862 when it was re-designated Swift-Tuttle.

The observation from 1862 showed that the meteors had virtually the same orbit as the comet and established the connection between these two types of object. The meteors are visible throughout the first half of August, but reach maximum numbers on August 12-13. On this date around 60 'shooting stars' may be seen each hour, and with the Moon virtually absent there will be little interference from moonlight.

The cometary particles enter the atmosphere at about 60 km/sec. At that velocity they are completely vapourised during their swift passage through the atmosphere. The meteors or 'shooting stars' follow paths that diverge from Perseus, it is not necessary to direct your attention to that particular constellation, since the meteors can appear in any part of the sky and it may be more productive to watch some of the nearby constellations such as Pegasus, Cassiopeia or Ursa Major.

Whilst watching for meteors, it is almost certain that the observer will spot some star-like objects moving rather more leisurely across the sky. These are some of the thousands of satellites and rockets that have been launched into orbit over the past 53 years.

From their height of several hundred kilometres above the Earth's surface they can still see (and reflect) sunlight making them visible when it is dark on the surface. Exactly 50 years ago, (1960 August 12) a huge inflated balloon satellite named Echo 1 was put into an orbit 1600 km. high. During its 8 year lifetime it became the man-made object seen by more people than any other. Has the International Space Station taken on this title?


 

Moon phases

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