The 2015 opposition of Saturn occurs in the constellation of Libra on May 23rd. Southern Hemisphere observers are favoured owing to the planet’s southerly declination, but on nights of calm seeing much detail will still be seen in modest instruments from the British Isles. Saturn is conveniently situated a little over 10° — the span of a fist at arm’s length — to the upper right of Antares in Scorpius and the Moon passes close by on Tuesday, June 2nd. The ringed planet is highest in the sky around 1 am BST on May 23rd, reaching this position half an hour earlier each passing week. AN graphic by Ade Ashford.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.
The relative sizes of Saturn and Earth compared, the ringed planet’s globe is about nine times larger than our own. The magnificent ring system (seen in part here) has a diameter of around 170,000 miles (270,000 kilometres) — equivalent to 70% of the distance from the Earth to the Moon. Image credit: NASA, via Wikimedia Commons.Physical properties
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.
A computer simulation of Saturn and its magnificent ring system as it will appear through a large telescope at opposition on May 23rd. The northern hemisphere of the planet is tipped in our direction by a favourable 24.4°, so the ring plane is well seen — look out for the Cassini Division, the dividing line between the A and B rings. Saturn’s southerly declination means that the planet will only attain a maximum altitude of 17° above the southern horizon for the centre of the British Isles, but with steady seeing and patience, much will still be seen in amateur instruments. Note that Newtonian reflectors will invert the image above. AN graphic by Ade Ashford/Stellarium.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?
Saturn’s moons Titan, Saturn’s largest moon, orbits its parent planet every 15.945 days. It’s easily identified in small telescopes as the 9th magnitude ‘star’ within four ring diameters of Saturn. It may be seen furthest east of the planet on May 23rd at 10 pm BST and furthest west of Saturn on May 31st at 8 pm BST. Note that this diagram shows north up and east to the left as it appears in the sky; users of Newtonian/Dobsonian reflectors and refractors/catadioptrics with a star diagonal will see east as right in the eyepiece. AN graphic by Ade Ashford/Stellarium.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.
A global ocean lies beneath the icy crust of Saturn’s geologically active moon Enceladus, according to new research using data from NASA’s Cassini mission. Researchers found the magnitude of the moon’s very slight wobble, as it orbits Saturn, can only be accounted for if its outer ice shell is not frozen solid to its interior, meaning a global ocean must be present.
If you’re an early riser in the British Isles, let the waning crescent Moon be your guide to three naked-eye planets – Mars, Jupiter and Saturn – at dawn on 18 and 19 March 2020. This celestial conjunction occurs in the constellation of Sagittarius where you can see all four Solar System bodies within the span of a fist at arm’s length. Look for attractive binocular conjunctions too.
