Mercury’s prominent evening show under southern skies

By Ade Ashford

Innermost planet Mercury is already putting on a good show for Southern Hemisphere skywatchers, low in the west-northwest in deep twilight an hour after sunset. However, Australasian observers are in for a treat on the evening of Tuesday, 25 July when the slim 5 percent crescent of a 2-day-old Moon lies in the same binocular field of view as magnitude +0.3 Mercury and first-magnitude star Regulus in Leo at the onset of nautical twilight. This shows the view from the New Zealand capital, but by the time nautical dusk falls on the east coast of Australia, the Moon lies just to the right of magnitude +5.3 star nu (ν) Leonis, somewhat closer to Mercury and Regulus. AN graphic by Ade Ashford/Stellarium.

Innermost planet Mercury, a rocky little world just one-third the diameter of the Earth, loops around the Sun in a decidedly eccentric (oval-shaped) orbit every 88 days or so. The time when it is most likely to be seen with the naked eye is at elongation — either low in the western sky shortly after sunset, or in the eastern sky during dawn twilight — when its angular separation from the solar disc reaches a peak. At such times Mercury can be anything from 18 to 28 degrees from the Sun.

Mercury currently lies in the constellation of Cancer, passing into Leo on 16 July. The planet is, therefore, an evening object. Owing to a favourable orbital geometry, Mercury’s separation from the Sun is presently an angle of 21 degrees (about the span of an outstretched hand at arm’s length) — but the gap is still widening. In fact, Mercury’s evening show continues to improve until it attains a maximum easterly elongation of 27 degrees on Sunday, 30 July.

Mercury from the UK and USA
However, before you all dash to ready your telescopes shortly after sunset to grab a glimpse of Mercury in the rosy glow of twilight, one needs to factor latitude into the equation. A line drawn between Mercury and the Sun makes a shallow angle to the western horizon as seen from the UK, so by the end of this month the planet sets just 45 minutes after the Sun, lost from view in the bright twilight. The view from North America is somewhat better where, for example, Mercury sets 1¼ hours after the Sun seen from Los Angeles, CA on 30 July. The Southern States will get the best views.

How latitude makes a huge difference to Mercury’s visibility at elongation. The diagram shows the innermost planet at its maximum 27-degree separation from the Sun on 30 July when the latter is 12 degrees below the horizon after sunset, a period known as nautical dusk. Note that as seen from the UK, Mercury is already 10 degrees below the horizon, having set over an hour ago. As seen from NZ, however, more of Mercury’s 27-degree elongation translates into height above the horizon, so the planet remains on view for a further 1¼ hours after nautical dusk. AN graphic by Ade Ashford.

Equatorial and Southern Hemisphere view
For observers on or south of the Earth’s equator, the line joining Mercury and the Sun is almost perpendicular to the west-northwest horizon at sunset, so the planet’s elongation translates into height above the horizon, significantly improving the duration of its visibility at civil dusk (when the Sun lies 6 degrees below the horizon and planets/bright stars first become visible). By the third week of July 2017, Mercury sets 1⅔ hours after sunset as seen from Singapore, or slightly more than 2 hours after sunset as seen from Sydney, Australia.

The last week of this month offers a Mercury viewing bonanza for antipodean astronomers. At elongation on 30 July, Mercury sets almost 2¼ hours after sunset as seen from Sydney, NSW or an amazing 2h 20m after sunset as viewed from the New Zealand capital. In fact, as viewed from both Australia and New Zealand, Mercury can be seen in a dark sky some 8 degrees high in the west-northwest at the end of astronomical twilight by the end of the month.

However, real treat lies in store for Southern Hemisphere observers on the evening of Tuesday, 25 July. At nautical dusk (~1 hour after sunset in NZ), magnitude +0.3 Mercury lies 1¼ degrees from first-magnitude star Regulus in the constellation of Leo with the slim crescent of a 2-day-old Moon hanging 2⅔ degrees below the planet. This beautiful celestial juxtaposition is easily encompassed by the field of view of a typical 10×50 binocular (see graphic at the top of the page).

Mercury hidden by the Moon
Returning to the Northern Hemisphere for a moment, the young Moon occults Mercury on the morning of Tuesday, 25 July as seen from the UK. (Disappearance: 8:33am London, 8:29am Edinburgh; reappearance: 8:53am London, 9:10am Edinburgh. All times BST.) Mercury’s disc is just 7.1 arcseconds wide and 54 percent illuminated at this time. Since this all occurs very low in the eastern sky in full daylight for the UK it will be extremely challenging to observe. The occultation is also visible from most of Greenland, Northern Europe and the northern half of Asia.

Wherever you may live, I make no apologies for expressing the usual warning to never casually sweep for Mercury with a binocular or telescope when the Sun is still above the horizon. Unless you are an experienced observer, please confine your observations of Mercury to before sunrise or after sunset, according to the apparition. Sunrise and sunset times, plus observing data for the Moon and planets for anywhere in the world is readily available through our interactive Almanac.