March 2015 update: Even a casual stargazer cannot fail to notice the bright ‘star’ that’s currently high to the south-southeast as evening twilight fades, an object exceeded in brightness only by Venus on the western horizon at dusk, or the Moon. Patient observation over a week or more will reveal that the eastern luminary is slowly moving against the background stars, betraying its planetary nature — it is, of course, Jupiter.
Jupiter is in the constellation of Cancer where it will remain until June this year. Despite being at opposition to the Sun on February 6th, it’s still relatively close and visible until the first signs of dawn twilight. The planet’s disc is 42 arcseconds in diameter, so a telescope employing a magnification of just 45x will make it appear the same size as our Moon to the unaided eye.
Jupiter is the Solar System’s largest planet with a mass exceeding that of all the other planets combined. Eleven times the diameter of our planet, more than 1,300 Earth’s could comfortably fit within its enormous bulk. Jupiter is a body largely composed of hydrogen and helium under gaseous and liquid states that’s noticeably pot-bellied in appearance owing to a fast rotation period of under ten hours.A world with turbulent weather systems, Jupiter’s upper cloud layers provide an ever-changing tableau for owners of modest to large telescopes. The Great Red Spot (GRS) is an anticyclonic storm that has been observed for around 400 years. Our Almanac gives predictions for the best times you can see the GRS, as well as other phenomena relating to Jupiter’s largest moons. (As a quick summary for UK-based observers, the Great Red Spot is well placed for observation on the nights of 27th, 29th and 30th March 2015.)
As befits the King of the Planets, Jupiter is accompanied by an impressive retinue of 67 natural satellites at the last count. The largest of these by far are the so-called Galilean moons — Io, Europa, Ganymede and Callisto — discovered by Galileo Galilei in 1609/1610. Europa, the smallest of the ‘big four’, has a diameter slightly less than our own Moon, whereas the largest (Ganymede) exceeds planet Mercury in size.
Io, Europa, Ganymede and Callisto continually play cat and mouse as their orbital motion causes them to alternately pass in front of (transit) Jupiter or be hidden (occulted) by their parent planet or its shadow. Furthermore, the shadows of the Galilean moons frequently fall on the cloud tops of Jupiter, visible in modest telescopes of good quality as inky black dots slowly drifting across the face of the planet. Predictions for these shadow transits for any given date may also be obtained through our Almanac. Sometimes, multiple shadow transits occur. Sadly, there are no multiple shadow transits of the Galilean moons in March 2015.
Since Jupiter’s equator and the orbital planes of the Galilean moons are currently almost edge-on to our line of sight, we are able to witness somewhat rarer mutual events where the moons occult and eclipse each other — something that only happens twice in the planet’s 11.9-year orbit of the Sun. These events require a large telescope and good seeing to appreciate, but they are nonetheless fascinating to witness. Here are the events visible from the British Isles (weather permitting) throughout March 2015:Note: the times are given in Coordinated Universal Time (UTC), which is loosely synonymous with Greenwich Mean Time (GMT) unless very high accuracy (< 1 second) is required.
Inside the magazine
You can find out more about Jupiter in the March edition of Astronomy Now in addition to a full observing guide to the night sky.
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