Mercury at greatest elongation west

In the southern hemisphere Mercury will be well placed for observation in the dawn sky, shining brightly at mag 0.3.

From London however, it will not be observable – it will reach its highest point in the sky during daytime and will be 2° below the horizon at dawn.

Mercury’s orbit lies closer to the Sun than the Earth’s, meaning that it always appears close to the Sun and is very difficult to observe most of the time.

It is observable only for a few days each time it reaches greatest separation from the Sun – moments referred to as greatest elongation.

These apparitions take place alternately in the morning and evening skies, depending whether Mercury lies to the east of the Sun or to the west.

When it lies to the east, it rises and sets a short time after the Sun and is visible in early evening twilight. When it lies to the west of the Sun, it rises and sets a short time before the Sun and is visible shortly before sunrise.

On this occasion, it lies 27° to the Sun’s west.

Mercury in coming weeks

The key moments in this apparition of Mercury are as follows:

After greatest elongation, the distance between Mercury and the Sun will decrease each night as it sinks back into the Sun’s glare. The table below lists how long before sunrise Mercury will rise each night; all times are given in London local time.

A graph of the angular separation of Mercury from the Sun around the time of greatest elongation is available here.

Mercury’s position

The position of Mercury when it reaches greatest elongation will be:

The coordinates above are given in J2000.0.

Seasonal effects

At each apparition, Mercury reaches a similar separation from the Sun – around 18–28°. This distance is set by the geometry of how big Mercury’s orbit is, and how far away it is from the Earth.

Nonetheless, some times of the year are more favourable for viewing Mercury than others.

It appears most favourably in the evening sky around the time of the local spring equinox, and most favourably in the morning sky around the local autumn equinox.

These dates are reversed in the northern and southern hemispheres, such that a good apparition in one hemisphere will not be easily observable from the other.

This is comes about because Mercury always lies close to the line of the ecliptic, shown in yellow in the planetarium above. This is the line through the zodiacal constellations that the Sun follows through the year, and marks the flat plane in space in which all of the planets circle the Sun.

When Mercury rises a few hours ahead of the Sun, the altitude it reaches Mercury above the horizon before sunrise depends on two factors.

One is its angular separation from the Sun. But equally important is how steeply the line of the ecliptic is inclined to the horizon.

If Mercury is widely separated from the Sun along the ecliptic, this may not translate into a high altitude if the ecliptic meets the horizon at a very shallow angle, running almost parallel to it.

Conversely, if the ecliptic is almost perpendicular to the horizon, a much smaller separation from the Sun may place Mercury higher in the sky.

The inclination of the ecliptic plane to the horizon at London varies between 61° (sunrise at the autumn equinox) and 15° (sunrise at the spring equinox). On April 11, the ecliptic is inclined at 15° to the eastern dawn horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion Mercury is poorly placed for viewing from London.

This entry in the observing calendar was provided by In-The-Sky.org

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