Saturn, the Lord of the Rings, reaches opposition

If we exclude the Earth, the most beautiful planet in our solar system has to be the ringed world Saturn. March sees it come to opposition in the constellation Virgo, so that, like Mars a month ago, it stands opposite the Sun in the sky where it is visible throughout the night at its closest and brightest. Mars remains well placed, though, passing high in the south in the mid-evening and shining high in the south-west at our star map times.

Mars recedes from 116 million to 152 million km during March, its small ochre disk shrinking from 12 to 9 arcseconds in diameter and its brightness halving from magnitude -0.6 to 0.2. Tracking westwards against the stars of Cancer, it reaches a stationary point on the 11th when it resumes its more usual eastwards progress. Its easterly motion will continue until January 2012 when the Earth next begins to overtake Mars. Our chart depicts Mars about half way between Pollux, the brighter Twin of Gemini, and the Praesepe star cluster in Cancer.

Saturn reaches opposition on the 22nd when it lies 1,272 million km away in Virgo (see our south map), rises in the east at sunset, passes 36 high in the south in the middle of the night and sets in the west at sunrise.

At magnitude 0.6, Saturn is a little brighter than Virgo's leading star Spica and it would be brighter still were its ring system tipped more in our favour. As it is, we are now looking down on the north face of the rings, but only at an angle of 4 which, because of the Earth's motion, drops below 3 by the month's end.

The rings extend across 44 arcseconds at opposition, while Saturn's cloud-covered globe, is 20 arcseconds wide but flattened because of the planet's rotation in under 11 hours.

Several Saturnian moons are visible through a small telescope, with the largest, Titan, shining at the eighth magnitude and taking 16 days to circle the planet in the plane of the rings. Look for it furthest west of Saturn (almost 200 arc seconds) on the 3rd and 19th, and furthest east on the 11th and 27th. Saturn lies to the left of our Moon next Monday, above the Moon on Tuesday, left of the Moon again on the 28th and above the Moon once more on the 29th. Orion's retreat is hastened by the fact that the days are lengthening. Indeed, the Sun moves more than 12 northwards during March and crosses the sky's equator at 17:32 GMT on the 20th, the instant of the vernal equinox.

Sunrise/sunset times for Edinburgh change from 07:05/17:47 GMT on the 1st to 05:47/18:49 GMT (06:47/19:49 BST) on the 31st, after British Summer Times comes into play on the morning of the 28th. Nautical twilight lasts for about 80 minutes at dawn and dusk. The Moon is full on Sunday, at last quarter on the 7th, new on the 15th, at first quarter on the 23rd and full again on the 30th. Once again this month, the young Moon is quick to reappear in our evening sky and there is a chance that it might be glimpsed as the most slender of U-shaped crescents very low in the west just after sunset on the 16th, just 21 hours after new. On that evening, it lies 8 to the right of Venus and just a shade lower in the sky. Venus is brilliant at magnitude -3.9 and stands higher in the western twilight each day, its altitude 30 minutes after sunset improving from 4 to 11 during March. It is still near the Sun's far side, though, and shows a small almost-full disk only 10 arcseconds across if viewed through a telescope. After rounding the Sun's far side on the 14th, Mercury lies within 5 below and right of Venus during the final week of March but is considerably fainter near magnitude -1.0.

Look for the brightly earthlit crescent Moon on the evenings following the 16th though the earthshine will have faded by the time it stands 3 below the Pleiades on the 20th. The Moon climbs so quickly in the west from night to night because its motion against the stars anticipates that of the Sun over the coming months.

The ecliptic, the Sun's apparent annual path, is at its steepest in the west now and marks the plane of our solar system. The Moon and the other planets never diverge far from this plane so are always found in a region we call the zodiac which follows the ecliptic at our map times from Taurus in to west, though Gemini, Cancer and Leo to Virgo in the south-east.

Also following the ecliptic is the dim reflected glow of countless dust particles, meteoroids, which lie between the planets. This glow is brightest as the zodiacal light, a diffuse cone of illumination which tilts up along the ecliptic as the twilight fades. Under exceptionally dark, moonless conditions, this glow extends into a sky-circling zodiacal band with an oval brighter region, the gegenschein, directly opposite the Sun in the sky. I have glimpsed the gegenschein only rarely, and only at this time of the year when its glow emanates from the star-poor area to the south and east of Leo, well away from the competing glow of the Milky Way.