August showers – By Clive Jackson

WELCOME TO the August night sky. This is the month when much of Europe is on holiday and people have more time to spend outside during the warm summer nights. August 12 is the traditional peak of the Perseid Meteor Shower and, though not the best of the year, it is normally the most well-known. A meteor shower happens when the Earth passes through the orbit of a comet, which is long gone; in this case, a comet called Swift-Tuttle.  

Our Solar System has had thousands of comets orbiting the Sun since prehistoric times, all of them leaving behind a trail of dust and gas. The gas rapidly disperses, but the dust can stay around for centuries, slowly getting pushed out of orbit by the pressure of sunlight. These clouds of dust particles can bunch together sometimes and, when the Earth encounters this denser region, we experience a meteor storm with shooting stars.

Typically, these dust particles have a mass of less than one gram and this is sufficient to produce a brilliant streak of light in the night sky. Meteors of this size never make it to the ground, but occasionally we encounter a rock of several kilos mass, which produced a spectacular fireball in the sky, and survived to hit the ground. When this happens, the rock is called a meteorite.

Approximately 500 small meteorites hit the ground every year. In recent times there is no evidence of any fatalities due to meteorite impacts, but two cars have been hit. Maybe this proves that cars cover more square metres of the Earth surface than people do!

On August 1, the Quarter Moon was near the giant planet Jupiter and, in the early morning of August 10, Venus and Mercury will close together, which are visible low in the east. On August 22, the thin crescent Moon is near to both of them just before sunrise.

The Moon was at First Quarter on August 2, Full on August 9, Last Quarter on August 16, New on August 23, and First Quarter again on August 31.

• Clive Jackson is the director of the Astronomical Observatory of Tavira (Sitio do Malhão) and the Camera Obscura (next to the Castle in Tavira), specialising in education and public outreach. Tel: 281 321 754, fax: 281 324 688, e-mail: [email protected], visit