Saturn at opposition

Tuesday 9th Jul 201915:55

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, it will be difficult to observe as it will appear no higher than 16° above the horizon. It will be visible between 22:42 and 03:28. It will become accessible at around 22:42, when it rises 10° above your south-eastern horizon, and then reach its highest point in the sky at 01:07, 16° above your southern horizon. It will become inaccessible at around 03:28 when it sinks to 10° above your south-western horizon.

Saturn opposite the Sun

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 9.03 AU, and its disk will measure 18.4 arcsec in diameter, shining at magnitude 0.1. 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

Saturn will be angled to show its northern hemisphere at this opposition, and the rings will inclined at an angle of 24° to our line of sight, which is almost the maximum inclination they can have. This means they will be very well presented.

The graph below shows the changing inclination of Saturn’s rings over time. The black line indicates their inclination to our line of sight from the Earth. A negative angle indicates that the north pole is tipped towards us, while a positive angle indicates that we see the south pole. A angle close to zero means that Saturn’s rings appear close to edge on.

The red line indicates the inclination of the rings to the Sun’s line of sight to the planet. Interesting phenomena can occur when the rings are very close to edge-on, if the Sun illuminates one side of the rings, while we see the other. At such times, we see the unilluminated side of the rings.

The inclination of Saturn's rings around the time of its opposition in July 2019.

The Seeliger Effect

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.

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 2019 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 19h13m10s -22°02' Sagittarius 0.1 18.4"

The coordinates above are given in J2000.0.

This entry in the observing calendar was provided by