Jupiter at opposition

2015 Feb 6

Dominic Ford – originally published on In-The-Sky.org

Jupiter will be well placed for observation, in the constellation Cancer. It will be visible for much of the night, reaching its highest point in the sky at around midnight local time.

From London (click to change), it will be visible between 17:41 and 06:50. It will become accessible at around 17:41, when it rises 7° above your eastern horizon, and then reach its highest point in the sky at 00:17, 55° above your southern horizon. It will become inaccessible at around 06:50 when it sinks to 8° above your western horizon.

Jupiter in coming weeks

Over the weeks following its opposition, Jupiter will reach its highest point in the sky four minutes earlier each night, gradually receding from the pre-dawn morning sky while remaining visible in the evening sky for a few months.

A chart of the path of Jupiter across the sky in 2015 can be found here, and a chart of its rising and setting times here.

The position of Jupiter at the moment it passes opposition will be:

Object Right Ascension Declination Constellation Magnitude Angular Size
Jupiter 09h20m40s +16°31' Cancer -2.6 44.4"

The coordinates above are given in J2000.0.

The geometry of the solar system

This optimal positioning occurs when Jupiter is almost directly opposite the Sun in the sky. Since the Sun reaches its greatest distance below the horizon at midnight, the point opposite to it is highest in the sky at the same time.

At around the same time that Jupiter passes opposition, it also makes its closest approach to the Earth – termed its perigee – making it appear at its brightest and largest.

This happens because when Jupiter lies opposite the Sun in the sky, the solar system is lined up so that Jupiter, the Earth and the Sun form a straight line with the Earth in the middle, on the same side of the Sun as Jupiter.

In practice, however, Jupiter orbits much further out in the solar system than the Earth – at an average distance from the Sun of 5.20  times that of the Earth, and so its angular size does not vary much as it cycles between opposition and solar conjunction.

On this occasion, Jupiter will lie at a distance of 4.35 AU, and its disk will measure 44.4 arcsec in diameter, shining at magnitude -2.6. Even at its closest approach to the Earth, however, it is not possible to distinguish it as more than a star-like point of light with the naked eye, though a good pair of binoculars are sufficient to reveal it as a disk of light with accompanying system of moons.

The details of this observing event were provided courtesy of In-The-Sky.org