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For Northern Hemisphere observers October offers the best opportunity this year to spot Mercury - the smallest Solar System planet - in the morning sky. Since it's also the closest planet to the Sun it never ventures more than 28 degrees from our star and therefore is easily visible on just a few occasions each year. Even then almost always in twilight and never against a black midnight sky. So if you do have clear skies this month and live at northern or tropical latitudes, don't miss the opportunity to catch a glimpse of this elusive World.

Mercury as imaged by the MESSENGER space probe (credit - NASA)

This month Mercury reaches greatest western elongation on October 16th. It's visible as an early morning object from northern temperate locations from about the first week of October until the last few days of the month. The planet is also visible to a lesser extent from tropical regions but those in the Southern Hemisphere are out of luck, it's badly placed and not observable.

When searching for Mercury a clear unobstructed view of the horizon is ideal. Although the planet at the time of greatest elongation east is 18 degrees from the Sun, by the time the sky darkens it will be only a few degrees above the horizon. From mid-northern temperate latitudes, Mercury will reach 9 degrees above the east-southeastern horizon fourty-five minutes before sunrise on October 16th. Its altitude then decreases gradually each day until it's lost to the bright morning twilight at months end. However, the planet does brighten during the visibility period from mag. +1.3 on October 8th to mag. -1.0 on October 29th. In the same area of sky 25 degrees or so to the northwest are Venus (mag. -4.5), Jupiter (mag. -1.8) and Mars (mag. +1.8).

The charts below show Mercury's morning apparition from mid northern latitudes (e.g. London, England). A similar view exists at other northern temperate latitudes.

October morning apparition of Mercury from mid northern latitudes, 45 minutes before sunrise (credit:- freestarcharts)

Mercury, Venus, Jupiter and Mars from mid northern latitudes, 45 minutes before sunrise on October 16th (credit:- stellarium)

One interesting event occurs on October 11th when the waning crescent Moon passes just 0.9 degrees south of Mercury. An occultation is visible but you have to travel or reside in South S. America or the Falkland Islands to see it.

The current long morning apparition of Venus reaches a peak on October 26th when it attains greatest western elongation. On this day the planet will be positioned 46 degrees from the Sun and visible for over 4 hours before sunrise at mid Northern Hemisphere latitudes, although only about half as long from southern locations. The brilliant planet shines at mag. -4.4 and therefore is an unmistakable beacon of light blazing above the eastern horizon. In fact, so bright is Venus it's often reported to the police as a hovering UFO!

Venus in the early morning sky (credit:- freestarcharts)

What adds to the spectacle is that Venus is not the only planet visible in the early morning sky. Jupiter and Mars are located nearby and even elusive Mercury may by glimpsed at northern temperate latitudes. An added bonus is that on the same day as Venus reaches greatest elongation west it passes just 1.1 degrees south of Jupiter (mag. -1.8) with fainter Mars (mag. +1.7) positioned 5 degrees east of the pair. For comparison, Venus is 11 times brighter than Jupiter with Jupiter 25 times brighter than Mars! As previously mentioned, just before sunrise those at northern temperate will also be able to spot Mercury (mag. -0.9) a few degrees above the horizon.

The diagram/illustrations below show the positions of the planets on October 26, 2015.

Venus, Jupiter and Mars on October 26, 2015 (credit:- freestarcharts)

Venus, Jupiter and Mars on October 26, 2015 - pdf format (credit:- freestarcharts)

Venus, Jupiter and Mars as seen from mid northern latitudes on October 26th, 2 hours before sunrise (credit:- stellarium)

Venus, Jupiter and Mars as seen from mid southern latitudes on October 26th, 1 hour before sunrise (credit:- stellarium)

See also
The Planets this Month - October 2015