Mercury at greatest elongation west

Across much of the world Mercury will be well placed for observation in the dawn sky, shining brightly at mag -1.8.

From London (click to change) however, it will not be observable – it will reach its highest point in the sky during daytime and will be no higher than 0° above the horizon at dawn.

Mercury in coming weeks

Over coming weeks, the distance between Mercury and the Sun will decrease each morning, and it will gradually sink back into the Sun’s glare, as the table below indicates (all times given in London local time).

Mercury’s position

The exact position of Mercury when it reaches greatest elongation is as follows:

The coordinates above are given in J2000.0.

The geometry of Mercury’s orbit

Mercury never ventures far from the Sun in the sky since its orbit lies closer to the Sun than the Earth’s. To see it, we are always looking inwards towards the center of the solar system, close to the Sun. Mercury is said to be at greatest enlongation when it passes either of the two points along its orbit where it appears at greatest separation from the Sun.

As it orbits the Sun, it appears alternately in the morning and evening skies. When it is to the east of the Sun, it rises and sets a short time after the Sun, becoming visible in early evening twilight. When it is to the west of the Sun, it rises and sets a short time before the Sun, becoming visible shortly before sunrise.

Seasonal variation

Each time it appears in the morning or evening sky, Mercury reaches roughly the same angular separation from the Sun – this time peaking at a distance of 27° at greatest elongation. This angle 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 from London than others.

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

This is because it always lies close to a line across the sky called the ecliptic. 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.

The altitude at which Mercury appears above the horizon at sunrise or sunset depends how steeply the line of the ecliptic is inclined to the horizon. If the plane of the ecliptic meet the horizon at a very shallow angle, Mercury will rise or set along a line which is almost parallel to the horizon, and a separation of 27° from the Sun along this line would correspond to a very low altitude 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 29, the ecliptic is inclined at 17° 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.