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BAA Observing calendar

Mars at opposition

Friday, 2018, July 27 - 06:07

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

Mars will be well placed for observation, in the constellation Capricornus. 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 12° above the horizon. It will be visible between 23:02 and 03:20. It will become accessible at around 23:02, when it rises 7° above your south-eastern horizon, and then reach its highest point in the sky at 01:13, 12° above your southern horizon. It will become inaccessible at around 03:20 when it sinks to 8° above your south-western horizon.

Mars in coming weeks

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

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

Object Right Ascension Declination Constellation Magnitude Angular Size
Mars 20h31m30s -25°33' Capricornus -2.8 24.3"

The coordinates above are given in J2000.0.

The geometry of the solar system

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

The time of Mars's perigee is an especially good time to observe it, since it neighbors the Earth in the solar system and has the greatest variation of all of the planets in its distance from the Earth. This in turn leads to a large variation in its apparent size and brightness.

Mars's distance from the Earth can vary between 0.36 AU and 2.68 AU, meaning that its disk varies in diameter between 25.68" and 3.49".

When it passes opposition, Mars sweeps past the Earth rather quickly, and so only appears large and bright in the sky for a few weeks.

On this occasion, Mars will lie at a distance of 0.39 AU, and its disk will measure 24.3 arcsec in diameter, shining at magnitude -2.8. 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 details of this observing event were provided courtesy of In-The-Sky.org

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