Red Planet at brightest for 60,000 years

IT IS almost 60,000 years since Mars was as bright and close to the Earth as it gets this month.

Our star map plots Mars in the south-east, with its 6 of westerly (retrograde) motion in the constellation Aquarius shown by the arrow. High on the meridian at the map times is the Summer Triangle of Deneb, Vega and Altair. The Milky Way arches overhead and close to Deneb as it flows from Sagittarius and Scorpius on the south-south-western horizon towards the north-east, where Capella twinkles strongly in Auriga. It is all too easy, though, for the Milky Way’s misty band to be swamped by moonlight or the light pollution with which we surround (and submerge) ourselves.

The maps show the bright star Arcturus in Bootes sinking in the west as the Plough stands in the north-west and the Milky Way constellation of Cassiopeia, whose main stars form a distinctive "W", climbs high in the north-east. Andromeda lies a little lower in the east, its principal stars lying almost parallel to the horizon and below the famous Andromeda Galaxy, visible as a fuzzy oval to the naked eye and easier still through binoculars. This "big-sister" galaxy to our own Milky Way system lies some 2.5 million light years away.

The orange-red hue that makes Mars so striking and unmistakable to the unaided eye results from minerals of iron in its surface rocks and dust; indeed, it might have been called the Rusty Planet. Viewed through a telescope, the disk grows to 25 arcseconds in diameter at opposition, large enough for even small telescopes to show its indistinct markings. The darker areas were named maria (seas) by analogy with the Moon and were later thought to be areas of vegetation whose appearance varied with the Martian seasons as water flowed from the melting polar icecaps. Some observers, most notably Percival Lowell in the United States, believed they saw a web of canals constructed by the Martians to marshal their limited water resources.

Now we know that the canals were optical illusions and that the dark features represent regions of darker rocks whose appearances and outlines are modified by changes in the deposition of lighter-coloured, wind-blown dust. The various space missions - one Japanese, one European and two American - on their way to Mars, and due to arrive in December and January, may help to inform us whether the Martians themselves, or any form of life on Mars, may also be figments of our imagination.

As I remarked last time, global dust storms sometimes erupt when Mars passes through perihelion, its closest point to the Sun. Although this milestone is not reached until August 30th, the first major storm was sighted on July 1st. That storm subsided a few days later after blanketing an area about 1,500 km wide but more severe storms may develop at any time, perhaps to hide the entire surface beneath an orange veil. Meantime we have an unrivalled window in which to view Mars as never before, and to witness its white southern polar icecap dwindle away in the warmth of Mars’s southern hemisphere summer.

Viewed from Edinburgh this month, sunrise/sunset times change from 05:16/21:21 BST on the 1st to 06:14/20:11 on the 31st. Meanwhile, the duration of nautical twilight at dawn and dusk shrinks from 120 minutes to 89 minutes. The Moon is at first quarter an the 5th, full on the 12th, at last quarter on the 20th and new on the 27th.

The bright just-gone-full Moon stands to the right of Mars on the morning of the 13th as the Perseids meteor shower reaches its climax. There were strong displays by this annual shower during the 1990s, but rates have now declined to more typical levels of about 80 meteors per hour for an observer under ideal conditions. The strong moonlight this time, though, means that only the brightest meteors might be noticed as they diverge from a radiant point in northern Perseus, beneath Cassiopeia.

This point, marked on our northern star map, rises through the north-eastern sky overnight to stand very high in the east before dawn. The shower is already underway and peters out by the 20th.

Before dawn is also the time to look out for the only other bright planet visible this month. Saturn, which lies in western Gemini to the right of Castor and Pollux, rises at Edinburgh’s north-eastern horizon at 02:31 on the 1st and by 00:49 on the 31st. Though it is low in the morning twilight at present, by the month’s end it stands more than 30 high in the east before we lose it in the dawn rays. Fading slightly from magnitude 0.1 to 0.2, it is similar in brightness to Capella, which stands some 30 directly above. When it lies below the waning Moon on the morning of the 23rd, Saturn appears 17 arcseconds wide if viewed through a telescope, with the glorious rings 39 arcseconds across and tipped 25 to the Earth.

Venus, which reaches superior conjunction on the Sun’s far side on the 18th, and Jupiter, at conjunction on the 22nd, remain hidden in the Sun’s glare. Mercury is too low in the evening twilight to be seen. Neptune stands 4,261 million km away at opposition in Capricornus on the 4th and is a dim binocular object of magnitude 7.8. Uranus, brighter at magnitude 5.7, lies 2,846 million km away and stands 8 above-right of Mars when it reaches opposition in Aquarius on the 24th.