PICK OF THE MONTH
Jupiter
Best time to see: 19 August, 01:22 BST (00:22 UT)
Altitude: 23.5˚
Location: Capricornus Direction: South
Features: Detailed atmosphere, Great Red Spot, moons Recommended equipment: 75mm or larger
Jupiter reaches opposition on 19 August, the planet managing to attain a peak altitude of 23.5˚ in true darkness, due south, at 01:22 BST (00:22 UT). Full Moon, technically the Moon at opposition, appears 6.3˚ south-southwest of Jupiter on the night of 22 August.
Opposition describes a position in the opposite part of the sky to the Sun. At such times, planets appear brighter and larger than at other, non-opposition, times. In Jupiter’s case, at opposition it shines at mag. –2.9 and through the eyepiece of a telescope has an apparent diameter of 49 arcseconds, large enough to present some impressive detail.
Over the past years, Jupiter has been low as seen from the UK, as it tracked along the most southerly part of the ecliptic. This is now slowly changing, and the planet is gaining altitude with each passing year.
At opposition it’s in Capricornus, but right on the constellation’s eastern border with Aquarius. Technically, it starts the month in Aquarius, tracking west slightly through the month, crossing the Aquarius–Capricornus border to be within Capricornus on 19 August.
Jupiter reached equinox in early May and we can see a consequence of this on the morning of the 20th. At equinox, the Sun crosses the planet’s equatorial plane. The Galilean moons have orbits closely aligned to this plane, and Io will transit Jupiter’s disc on 20 August from 02:50 BST (01:50 UT) until 05:08 BST (04:08 UT). Near opposition, Io’s shadow virtually lines up with the moon in terms of Jovian longitude. The proximity to equinox makes the shadow virtually line up in latitude as well. A similar Io transit occurs on 21 August between 21:18 BST (20:18 UT) and 23:33 BST (22:33 UT).