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 47º and
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 10º above the horizon in a twilight sky. More favourable
conditions occur in southern latitudes as the maximum elongation of 28º 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 18º 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.
The above diagrams show Mercury rising when the Sun is still about 6º 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
The superior planets, with orbits larger than Earth´s, are at their brightest at opposition to the Sun rather than at inferior
||to Elongation West
|| 72 days|
||to Superior Conjunction
||to Elongation East
||to Inferior Conjunction
|| 72 days|
Dates of greatest elongations for the inferior planets:
April 18th - 28º - 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 - 19º - 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 - 21º - 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.
March 5th - 18º - 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 - 26º - very low in the north-west after sunset and too faint to be seen, Mercury is unsuitably placed for northern observors.
October 26th - 24º - the most favourable evening apparition of the year for observors in the southern hemisphere.
March 27th - 46º - 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.
January 9th - at 23º visible low in the south-eastern sky before dawn for observors in the northern hemisphere.
May 7th - at 27º the most favourable morning apparition for observors in the southern hemisphere. Unsuitably placed for northern observors.
September 3rd - at 18º the most favourable morning apparition for northern observors.
December 23rd - at 22º visible as an early morning object. For northern observors Mercury is low in the south-eastern sky at the start of twilight.
March 23rd - at 19º the most favourable evening apparition of 2011 for observors in tropical and northern latitudes.
July 20th - at 27º the most favourable evening apparition of 2011 for observors in the southern hemisphere. Unsuitably placed for northern observors.
November 14th - at 23º visible as an evening object for southern and tropical observors.
January 8th - at 47º in Libra at -4.4 magnitude. A brilliant morning object, rising four hours before the Sun for observors in northern temperate latitudes.
January 27th - at 25º visible low in the south-eastern sky before dawn for observors in the northern hemisphere.
May 6th - at 25º, unsuitably placed for northern observors.
September 19th - at 18º and the best morning apparition of 2010 for northern observors.
April 8th - at 19º visible in the western evening sky. The most favourable evening apparition of 2010 for northern observors.
August 7th - at 27º and the most favourable evening apparition of 2010 for southern observors.
December 1st - at 21.5º, unsuitably placed for northern observors.
August 20th - at 46º and -4.4 magnitude.
January 4th - at 19º visible as an early evening object.
April 26th - at 20º and the most favourable evening apparition of 2009 for northern observors.
August 24th - at 27º.
December 18th - at 20º.
February 13th - at 26º not visible in the northern hemisphere.
June 13th - at 24º, unsuitably placed for northern observors.
October 6th - at 18º and the best morning apparition of 2009.
January 14th - at 47º and -4.4 magnitude, setting several hours after the Sun.
January 22nd - at 19º visible as an evening object.
May 14th - at 22º visible low above the west-north-western horizon at the end of evening twilight.
September 11th - at 27º.
March 3rd - at 27º.
July 1st - at 22º.
October 22nd - at 18º, the most favourable morning apparition of the year.
March 22nd – at 28º not visible as below the horizon.
July 20th – at 20º rising 1.5 hours before the Sun.
November 8th – at 19º and –0.5 magnitude, rising 2 hours before the Sun.
February 7th – at 18º and –0.5 magnitude.
June 2nd – at 23º and 0.6 magnitude.
September 29th – at 26º but too low in the sky for observation.
October 28th – at 46º and –4.4 magnitude, rising about 4 hours before the Sun.
June 9th – at 46º and –4.3 magnitude, setting just over 2 hours after the Sun.