The August night sky

By CLIVE JACKSON [email protected]

Clive Jackson is the Director of the Astronomical Observatory of Tavira (Sitio do Malhão, Tavira) and the Camera Obscura (next to the Castle in Tavira), specialising in education and public outreach.

This is the month of the giant planet Jupiter. In fact, just after sunset over on the southeastern horizon, Jupiter can be seen rising in the constellation of Capricorn.

This planet is unmistakable as it is the brightest star-like object in that part of the sky. Through a pair of binoculars, or a small telescope, the four biggest Moons of Jupiter are easily visible. Galileo discovered these Moons 400 years ago in 1609.  On the night of August 6 the full Moon will be seen close to Jupiter.

If you care to get up very early during August at around 3 am, you will see high in the east the brilliant planet Venus. At this time, Venus is called the ‘morning star’ although it is of course a planet. Venus will be bright enough to cast a shadow this month.

On the night of August 12, we have the traditional peak of the Perseid Meteor Shower. This is made up from dust left behind that was in the tail of a comet called Swift-Tuttle that was discovered in the 19th century. In fact all periodic meteor showers are created in this way when an object (usually a comet) passes the orbit of the Earth and leaves behind dust and small rocks. You do not need a telescope to enjoy it, just clear skies and lots of patience.

Our solar system has had thousands of comets orbiting the Sun since prehistoric times and all of them leave behind a trail of dust and gas. The gas rapidly disperses, but the dust can stay around for centuries and only slowly gets pushed out of orbit by sunlight pressure. These clouds of dust particles can bunch together sometimes, and when the Earth encounters this denser region we can experience a meteor storm with more than one ‘shooting star’ visible per second. These dust particles have typically a mass of less than one gram and this is sufficient to produce a brilliant streak of light in the nighttime sky. Meteors of this size never make it to the ground; occasionally we encounter a rock of several kilos mass and this is sufficient to produce a spectacular fireball in the sky, and it may survive to hit the ground. When this happens the rock is called a meteorite. Meteors normally are best seen after midnight but unfortunately the last quarter Moon rises at that time on August 12 – 13, making the night sky too bright to see faint meteors.

On August 27 the first quarter Moon will be seen to pass very close to the red giant star Antares in the constellation of Scorpios. It is possible that the star will disappear for a few minutes emerging once again on the bright side of the Moon.

The Moon is Full on August 6 and Last Quarter on August 13 and New on August 20, and First Quarter on August 27.

Clive Jackson can be contacted by phone on (00351) 281 321 754, by fax on (00351) 281 324 688, or by email to [email protected]. To visit his website, click on the link to the right of this page.