Night skies in Spain
Axarquia, Costa del Sol, Andalucia, Spain

ELONGATIONS

Relative positions

Elongation is a term that refers to the angle between the Sun and a planet as viewed from Earth. The best times to observe an inferior planet (Venus and Mercury which orbit the Sun inside the Earth’s orbit), is when they are close to greatest elongation. They are always seen as morning objects near western elongation or evening objects near eastern elongation. Venus has a maximum elongation of 47║ and Mercury 28║.

The greatest brilliancy of Venus occurs about 36 days before or after inferior conjunction. This is about a month after greatest eastern elongation as an evening object or a month before western elongation as a morning object. The distance of Mercury from the Earth and from the Sun varies over a wide range and the same rule does not apply.

In the northern hemisphere Mercury can be difficult to observe unless the horizon is clear. It is rarely as much as 10║ above the horizon in a twilight sky. More favourable conditions occur in southern latitudes as the maximum elongation of 28║ from the Sun, when Mercury is at aphelion (furthest from the Sun), places the planet south of the equator. For northern observors elongations can be as little as 18║ at perihelion (closest to the Sun). In general, the best time for viewing Mercury as an evening object is during spring before greatest eastern elongation or as a morning object during autumn after greatest western elongation.

The angle of the ecliptic to the local horizon varies at its rising or setting point, depending on latitude, the time of day and the time of the year. If Mercury or Venus is positioned in a constellation in which the ecliptic presents a shallow angle to the horizon when it is rising or setting, the planet will be very low down in the twilight and will only be visible for a short while, if at all. A steep ecliptic angle to the horizon, however, means the planet will be seen higher up in the twilight, resulting in a longer period of visibility. Therefore an inferior planet, even when it reaches greatest elongation, can appear low down in the twilight in certain seasons, but high up during others, either before sunrise or after sunset.

Mercury in the dawn sky
The above diagrams show Mercury rising when the Sun is still about 6║ below the horizon. They show the eastern morning sky at a northern and southern latitude in mid-April (Spring and Autumn respectively). Mercury is shown near greatest Western elongation, positioned slightly south of the ecliptic.

At latitude 50░ North, about 36 minutes before sunrise, the shallow angle of the ecliptic to the horizon causes Mercury to languish low in the sky, around 2░ in altitude. It is barely detectable in the bright twilight and is caught in the haze near the horizon. The planet has only been above the horizon for about 15 minutes.

At latitude 35░ South, about 26 minutes before sunrise, the steep angle of the ecliptic to the horizon ensures that Mercury appears high in the sky around 18░ in altitude, easily detectable against a darker sky. The planet has already been above the horizon for about an hour and a half.

The stars, planets and other celestial bodies move across the sky from east to west in a direction parallel to the celestial equator. The angle of the celestial equator to the local horizon remains fixed throughout the year and is determined by latitude. However, the angle of the ecliptic against the local horizon varies throughout the day and time of year. This variation applies at both eastern and western horizons, hence for any latitude, Mercury can be better observed in some seasons than others.

Venus is the brightest planet in our sky and may sometimes be spotted in daylight. As Mercury, it is highest in the sky during spring as a evening object and autumn as a morning object.

The superior planets, with orbits larger than Earth┤s, are at their brightest at opposition to the Sun rather than at inferior conjunction.

  Mercury Venus
Inferior Conjunction to Elongation West 22 days   72 days
Elongation West to Superior Conjunction 36 days 220 days
Superior Conjunction to Elongation East 35 days 220 days
Elongation East to Inferior Conjunction 22 days   72 days

Dates of greatest elongations for the inferior planets:

2012
Mercury western:
April 18th - 28║ - the most favourable morning apparition of 2012 for observors in the southern hemisphere. For northern observors, the planet is lost in the bright dawn glow.
August 16th - 19║ - at magnitude +0.1 - not a particularly favourable apparition for any latitude although observors in the tropics and north of the equator will have the best view. For northern observors, it is visible for a few mornings before and after elongation and, on the morning of the 16th, Mercury is just to the upper left of the crescent Moon.
December 4th - 21║ - magnitude -0,4 - not well placed for observors in the southern hemisphere but, because of Mercury┤s proximity to Venus at this time, an interesting apparition for those in the northern hemisphere.
Mercury eastern:
March 5th - 18║ - Mercury sets an hour and fourty-five minutes after the Sun at this most favourable evening apparition of the year for observors in the northern hemisphere.
July 1st - 26║ - very low in the north-west after sunset and too faint to be seen, Mercury is unsuitably placed for northern observors.
October 26th - 24║ - the most favourable evening apparition of the year for observors in the southern hemisphere.
Venus eastern:
March 27th - 46║ - a spectacular object in the evening sky, at -4.2 magnitude, setting over four hours after the Sun for observors in northern temperate latitudes. From the tropics and more southerly latitudes it is visible for less than half this time.

2011
Mercury western:
January 9th - at 23║ visible low in the south-eastern sky before dawn for observors in the northern hemisphere.
May 7th - at 27║ the most favourable morning apparition for observors in the southern hemisphere. Unsuitably placed for northern observors.
September 3rd - at 18║ the most favourable morning apparition for northern observors.
December 23rd - at 22║ visible as an early morning object. For northern observors Mercury is low in the south-eastern sky at the start of twilight.
Mercury eastern:
March 23rd - at 19║ the most favourable evening apparition of 2011 for observors in tropical and northern latitudes.
July 20th - at 27║ the most favourable evening apparition of 2011 for observors in the southern hemisphere. Unsuitably placed for northern observors.
November 14th - at 23║ visible as an evening object for southern and tropical observors.
Venus western:
January 8th - at 47║ in Libra at -4.4 magnitude. A brilliant morning object, rising four hours before the Sun for observors in northern temperate latitudes.

2010
Mercury western:
January 27th - at 25║ visible low in the south-eastern sky before dawn for observors in the northern hemisphere.
May 6th - at 25║, unsuitably placed for northern observors.
September 19th - at 18║ and the best morning apparition of 2010 for northern observors.
Mercury eastern:
April 8th - at 19║ visible in the western evening sky. The most favourable evening apparition of 2010 for northern observors.
August 7th - at 27║ and the most favourable evening apparition of 2010 for southern observors.
December 1st - at 21.5║, unsuitably placed for northern observors.
Venus eastern:
August 20th - at 46║ and -4.4 magnitude.

2009
Mercury eastern:
January 4th - at 19║ visible as an early evening object.
April 26th - at 20║ and the most favourable evening apparition of 2009 for northern observors.
August 24th - at 27║.
December 18th - at 20║.
Mercury western:
February 13th - at 26║ not visible in the northern hemisphere.
June 13th - at 24║, unsuitably placed for northern observors.
October 6th - at 18║ and the best morning apparition of 2009.
Venus eastern:
January 14th - at 47║ and -4.4 magnitude, setting several hours after the Sun.

2008
Mercury eastern:
January 22nd - at 19║ visible as an evening object.
May 14th - at 22║ visible low above the west-north-western horizon at the end of evening twilight.
September 11th - at 27║.
Mercury western:
March 3rd - at 27║.
July 1st - at 22║.
October 22nd - at 18║, the most favourable morning apparition of the year.

2007
Mercury western:
March 22nd – at 28║ not visible as below the horizon.
July 20th – at 20║ rising 1.5 hours before the Sun.
November 8th – at 19║ and –0.5 magnitude, rising 2 hours before the Sun.
Mercury eastern:
February 7th – at 18║ and –0.5 magnitude.
June 2nd – at 23║ and 0.6 magnitude.
September 29th – at 26║ but too low in the sky for observation.
Venus western:
October 28th – at 46║ and –4.4 magnitude, rising about 4 hours before the Sun.
Venus eastern:
June 9th – at 46║ and –4.3 magnitude, setting just over 2 hours after the Sun.

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