Mercury at greatest elongation west
|Sun, 05 Jun 2016 at
||(2 days away)
Dominic Ford, Editor
From the Inner Planets feed
Mercury will be well placed for observation in the dawn sky, shining brightly at mag -1.9.
From Virginia Beach (click to change), it will be difficult to observe as it will appear no higher than 8° above the horizon. It will be visible in the dawn sky, rising at 04:36 (EDT) – 1 hour and 8 minutes before the Sun – and attaining an altitude of 8° above the eastern horizon before fading from view as dawn breaks at around 05:24.
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 Virginia Beach local time).
|Altitude of Mercury
|Direction of Mercury
|29 May 2016
|05 Jun 2016
|12 Jun 2016
|19 Jun 2016
|26 Jun 2016
|03 Jul 2016
|10 Jul 2016
|17 Jul 2016
|24 Jul 2016
|31 Jul 2016
|07 Aug 2016
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.
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 24° 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 Virginia Beach 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 24° 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 Virginia Beach varies between 76° (sunrise at the autumn equinox) and 29° (sunrise at the spring equinox). On June 5, the ecliptic is inclined at 43° to the eastern dawn horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion Mercury is very favourably placed for viewing from Virginia Beach.
The circumstances of this event were computed using the DE405 planetary