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Mercury at maximum elongation, 25 May 2014

May 13, 2014 1 comment

The planet Mercury is the most elusive of all of the naked eye planets. It orbits nearest the Sun, and so always rises just before the Sun or sets just after it, appearing in the glow of twilight. For much of Mercury’s orbit it isn’t visible at all, lying too close to the Sun in the sky.

To see Mercury at its best you have to wait until it’s as far as possible from the Sun in the sky; what astronomers refer to as its maximum elongation. When Mercury is at its maximum eastern elongation it’s visible just before sunrise; when it’s at its maximum western elongation its visible just after sunset.

At the moment Mercury is nearing its maximum western elongation and so makes a perfect evening target.

Mercury in the WNW, at Maximum Elongation, 25 May 2014

Mercury in the WNW, at Maximum Elongation, 25 May 2014, created using Stellarium

Mercury’s range of maximum elongation is between 18° and 28°, and in this particular apparition it’s furthest distance from the Sun is 22.7°. This occurs on 25 May 2014. Between now and the end of May look west just after sunset to try and catch a glimpse of Mercury shining at magnitude +0.4. It’ll be low in the sky, very low, but if you look towards the west, find Jupiter shining brilliantly, and follow a line down to the right at an angle of approx. 45° you should see Mercury a few degrees above the horizon.

If you’re trying to observe it through a telescope then make sure you wait until the Sun has well and truly set below the horizon. Mercury exhibits phases like the Moon and Venus which can be seen through a telescope but shows no other detail through an earth-based scope; on 25 May the disk of Mercury facing the Earth will only be 40% illuminated, making a fat crescent shape. Mercury’s angular size is the smallest of all the planets save distant Uranus and Neptune.

If you’ve ever seen Jupiter or Saturn through a telescope then you’ll know that they look spectacular despite their relatively meagre size. On 25 May, for example, Jupiter will appear to have a diameter of 33 arcseconds (written 33″), Saturn 19″, Venus 15″, Mars 12″, and Mercury a paltry 8″.

And you can actually see all five of these planets on the night of 25 May (or any night between now and the end of May. Mercury is the trickiest to find, but Jupiter will be blazing low in the east, Mars high in the south, Saturn lower in the south-east, and if you’re keen to get up before sunrise you’ll see Venus low in the east. (Uranus and Neptune are dawn objects too at the moment).

Morning Mercury, December 2012

December 2, 2012 1 comment

Over the next few mornings you’ll be able spot the most elusive of the naked-eye planets, Mercury, low in the south-east just before sunrise.

Mercury is hard to find, and most days isn’t visible at all. Since it orbits so close to the Sun, when seen from Earth it never appears very far from the Sun in the sky. You can only catch it for a few days at a time when it’s furthest from the Sun in our sky, at a point called its maximum elongation. And even then it’s not that simple to find, as it will always be quite low on the horizon, hidden amongst twilight.

As Mercury whizzes round the Sun (it takes 88 days to make one complete orbit) sometimes we see it in the morning and sometimes in the evening. The amount of time between one morning appearance and the following evening appearance is around six or seven weeks. However Mercury isn’t very clearly visible at every maximum elongation (in some the Sun is much nearer the horizon so the sky is much brighter, making it harder to find), and even when it is clearly visible you’ll only catch sight of it on the few days before and after the date of maximum elongation.

Mercury’s next maximum elongation in of 4 Dec 2012, when it’s quite far (21°) west of the Sun, and quite bright (magnitude -0.3) making it quite easy to spot over the next few mornings.

How to find Mercury

If you have clear skies, head outside around 0630 and find somewhere with a good clear SE horizon (Mercury rises around 0630 and only gets a few degrees above the horizon by the time the Sun’s light begins to significantly brighten the sky).

Luckily there are two other planets up near Mercury right now, namely Venus and Saturn. Both of these planets are brighter than Mercury and higher in the sky, and together all three form a straight line leading diagonally down to the horizon. Find brilliant Venus, the brightest thing in the sky except for the Sun or the Moon, and then look for Saturn up and to the right, and Mercury in the opposite direction, down and to the left.

This photo, taken by the excellent Paul Sutherland, shows how the three planets lined up this morning (2 Dec) when viewed from the UK.

Mercury, Venus and Saturn in the morning sky. Image credit Paul Sutherland.

Mercury, Venus and Saturn in the morning sky. Image credit Paul Sutherland.

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