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BAA Observing calendar

Saturn at opposition

Wednesday, 2018, June 27 - 14:15

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

Saturn will be well placed for observation, in the constellation Sagittarius. 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 difficult to observe as it will appear no higher than 16° above the horizon. It will be visible between 23:02 and 03:02. It will become accessible at around 23:02, when it rises 11° above your south-eastern horizon, and then reach its highest point in the sky at 01:04, 16° above your southern horizon. It will become inaccessible at around 03:02 when it sinks to 11° above your south-western horizon.

Saturn in coming weeks

Over the weeks following its opposition, Saturn 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 Saturn across the sky in 2018 can be found here, and a chart of its rising and setting times here.

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

Object Right Ascension Declination Constellation Magnitude Angular Size
Saturn 18h24m10s -22°28' Sagittarius 0.8 18.4"

The coordinates above are given in J2000.0.

The geometry of the solar system

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

In practice, however, Saturn orbits much further out in the solar system than the Earth – at an average distance from the Sun of 9.56  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, Saturn will lie at a distance of 9.05 AU, and its disk will measure 18.4 arcsec in diameter, shining at magnitude 0.8. 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 without the aid of a telescope.

The rings of Saturn

The inclination of Saturn's rings around the time of its opposition in June 2018.

For a few hours around the exact moment of opposition, it may be possible to discern a marked brightening of Saturn's rings in comparison to the planet's disk, known as the Seeliger Effect. This occurs because Saturn's rings are made of a fine sea of ice particles which are normally illuminated by the Sun at a slightly different angle from our viewing angle, so that we see some illuminated particles and some which are in the shadow of others. At around the time of opposition, however, the ice particles are illuminated from almost exactly the same direction from which we view them, meaning that we see very few which are in shadow.


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

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