The one planet guaranteed to elicit an exclamation of wonder when first seen through a telescope is Saturn, particularly now that its glorious ring system is so prominently displayed. And it doesn’t require a large telescope either — a good quality 60mm refractor is more than adequate to distinguish Saturn’s globe and encompassing rings. As is usually the case, the larger the telescope the better, and a quality 6-inch refractor or 10-inch reflector on a good night will deliver views that will remain etched in the memory.
Sixth planet from the Sun and second only to Jupiter in size, Saturn is a gas giant composed mainly of hydrogen and helium in various states surrounding a rocky and metallic core, much like planetary siblings Jupiter, Uranus and Neptune. Saturn’s globe, some nine times the diameter of Earth, is noticeably bulged at the equator owing to the planet’s fast rotational period of 10.5 hours, so its equatorial and polar radii differ by almost 10%. The planet orbits the Sun every 29.46 years at a distance averaging 9.6 times the radius of Earth’s orbit.
The planet’s glorious ring system is composed of countless millions of icy moonlets ranging in size from specks of dust to around 10 metres in diameter, all orbiting Saturn in a 20-metre-thick plane, extending from 4,100 miles (6,600 kilometres) to 75,000 miles (120,700 km) above the planet’s equator. The ring system has a overall diameter of around 170,000 miles (~270,000 kilometres), equivalent to 70% of the distance between the Earth and Moon.
Observing the Ringed Planet
Saturn is at opposition to the Sun on May 23rd this year, so the planet will be closest to Earth and at its brightest in late spring and early summer. The planet passed from Scorpius into Libra on May 12th where it will remain until October 17th (when it returns to Scorpius). Given the planet’s southerly declination, this will be a relatively short apparition for observers at high northerly latitudes. As seen from the British Isles, Saturn will be highest in the sky at 12:30 am BST by the end of May, culminating half an hour earlier with each passing week.
Saturn’s equator currently spans about 18.5 arcseconds, so a telescope employing a magnification of 100x will make it appear the same size as the Moon to the naked eye. The major axis of the rings spans 42 arcseconds for comparison. At favourable ring plane presentations such as this and when the seeing is particularly steady, you may care to look out for the Cassini Division — the 3,000-mile (4,800-mile)-wide division between the A and B rings. What’s the smallest telescope that you can see it clearly with?
If you notice some starlike points of light close to Saturn in your telescope, these will be the planet’s moons. At the last count, the planet had 53 formally named moons, but there may be 100 more. The brightest is 9th magnitude Titan closely followed by 10th magnitude Rhea. On May 23rd/24th, Titan will lie slightly more than four ring diameters to the east of Saturn, which is to the right of the planet in Newtonian telescopes and refractor/Schmidt-Cassegrain/Maksutov-Cassegrain telescopes with a star diagonal. Titan orbits Saturn every ~16 days, so it will be seen close to western elongation of Saturn on May 31st/June 1st. Clear skies!
Inside the magazine
You can find out more about observing Saturn in the May edition of Astronomy Now in addition to a full observing guide to the night sky.
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