In the early evening of Tuesday, 23 February, the rising waning gibbous Moon — one day after full Moon — lies just 3½ degrees from magnitude -2.5 planet Jupiter low in the eastern sky. The pair can be viewed simultaneously in the field of view of most binoculars against the constellation backdrop of Leo. The Moon’s size has been slightly exaggerated for clarity. AN illustration by Ade Ashford.Even casual UK stargazers cannot fail to notice the bright yellow ‘star’ low in the east soon after 8pm GMT, an object that is more than twice the luminosity of brightest nighttime star Sirius sparkling low in the southern sky. Careful observation of the interloper in the east reveals that it is moving slowly with respect to the background stars of Leo from night to night; the object is, of course, Jupiter — the largest planet in the solar system.
Jupiter is more massive than of all the other known planets of the Sun’s family 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.Click the graphic to see a colour movie of Jupiter from NASA’s Cassini spacecraft showing what it would look like to peel off the entire globe of Jupiter, stretch it out on a wall into the form of a rectangular map, and watch its atmosphere evolve with time. The 14-frame animation spans 24 Jovian days, or about 10 Earth days, accelerated by a factor of 600,000. The wide equatorial zone is visible in the centre surrounded by two dark equatorial belts (SEB and NEB). The Great Red Spot is at the southern margin of the SEB. The smallest features visible at the equator are about 600 kilometres across. Image credit: NASA/JPL/University of ArizonaA world with turbulent weather systems, Jupiter’s upper cloud layers provide an ever-changing tableau for owners of six-inch (15-centimetre) telescopes and larger when the planet is highest in the sky (currently about 1:30am GMT for the centre of the British Isles) and seeing conditions are good. 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 a quick summary for UK-based observers, the GRS is well placed for observation on the nights of 19, 23, 26 and 28 February 2016.)
Jupiter is at opposition to the Sun on the morning of Tuesday, 8 March and closest to Earth shortly after 6pm GMT the same day. At its nearest, the planet will be 4.435 astronomical units, or 412.3 million miles (663.5 million kilometres) from Earth. Jupiter is therefore still approaching the Earth and its apparent equatorial diameter breaks the 44-arcsecond barrier in the third week of February 2016. This means that a telescope employing little more than 42x magnification is required to enlarge Jupiter to the same apparent size as the full Moon seen with the unaided eye.
A montage of Jupiter’s four Galilean moons, in a composite image comparing their sizes and that of Jupiter and the Great Red Spot. From top to bottom: Io (diameter 2274 miles), Europa (1940 miles), Ganymede (3270 miles), Callisto (diameter 2995 miles). Image credit: NASA/JPL/DLR – NASA planetary photojournalAs 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. The Galilean moons are so conspicuous that quality binoculars will reveal them when furthest from their parent planet.
Io, Europa, Ganymede and Callisto continually play cat and mouse as their orbital motion cause 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 (3-inch, or 7½-centimetre aperture) telescopes of good quality at magnifications of 100x or more as inky black dots slowly drifting across the face of the planet.On the night of Monday, 29 February, two of Jupiter’s Galilean moons — Io and Europa — perform a simultaneous shadow transit that is visible (weather permitting) from the British Isles. The shadow of Europa crosses Jupiter’s cloud tops from 8:33pm to 11:20pm, while Io’s shadow drifts across the face of its parent planet from 10:34pm to 12:50am (1 March). Therefore, the shadows of both moons will be seen on Jupiter from 10:34pm to 11:20pm (all times GMT). AN graphic by Ade Ashford.Predictions for the beginning and end times of Galilean shadow transits, plus information on their eclipses and occultations for any given date, may also be obtained through our Almanac. Sometimes, multiple shadow transits occur. For example, simultaneous shadow transits of Io and Europa visible from the UK occur on 22 and 29 February. For help using the Almanac, see this article.
Inside the magazine
Find out everything you need to know about observing Jupiter at its best and the other planets in the night sky in the March 2016 edition of Astronomy Now.
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.
Data from NASA’s Juno probe indicates violent electrical storms at higher altitudes than previously thought, storms that generate ammonia-enriched “mushballs” that fall like hail into the atmosphere below.
Skywatchers in the UK and Western Europe should look low to the south-southeast an hour before sunrise on 31 January to see a beautiful naked-eye conjunction of Venus, the old crescent Moon and Jupiter, all within a span of 8½ degrees. But if you have a telescope and live in just the right place, you can also see the Moon hide a double star.
