Three of Saturn’s moons — Tethys, Enceladus and Mimas — are captured in this group photo from NASA’s Cassini spacecraft. Tethys appears above the rings, while Enceladus sits just below centre and Mimas hangs below and to the left of Enceladus. This view looks toward the sunlit side of the rings from narrowly above the ring plane.
You might think that, in the rings of Saturn, more opaque areas contain a greater concentration of material than places where the rings seem more transparent. But this intuition does not always apply, according to a recent study of the rings using data from NASA’s Cassini mission. The research also suggests that the planet’s brightest B ring could be a few hundred million years old instead of a few billion.
At 6am GMT on the mornings of 3 and 4 February, around the onset of astronomical twilight for the centre of the British Isles, the old waning crescent Moon brushes by ringed planet Saturn low to the south-southeast horizon. As a bonus for telescope users, the Moon occults globular cluster M9 shortly after 6am GMT on 4 February too.
As avid skywatchers will already know, all of the bright naked-eye planets are currently visible in the pre-dawn sky — the first time in eleven years that such an alignment has occurred. At 6am GMT on Monday, 1 February, the last quarter Moon in the constellation Libra lies just 2½ degrees from magnitude +0.8 planet Mars low in the south for UK observers.
All five of the bright naked-eye planets are observable in the pre-dawn sky from about the third week of January 2016, particularly if one lives south of the equator. But even from the UK, you can get to view the spectacle if you time it right — and the weather obliges! The last time that Mercury, Venus, Mars, Jupiter and Saturn appeared in the same sky was 11 years ago.
Janus and Tethys demonstrate the main difference between small moons and large ones; it’s all about their shape. Moons like Tethys are large enough that their own gravity is sufficient to overcome the material strength of the substances they are made of and mould them into spherical shapes, but small moons like Janus are not massive enough for their gravity to form them into a sphere.
On the morning of Saturday, 9 January — just two days after their photogenic conjunction with an old crescent Moon — planets Venus and Saturn reach the denouement of their pre-dawn show with a spectacular close conjunction. To observe this spectacle you need an unobstructed view low to the southeast around 7am GMT (central British Isles).
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