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.

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:

2010
Mercury western:
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.
Mercury eastern:
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.
Venus eastern:
August 20th - at 46º and -4.4 magnitude.

2009
Mercury eastern:
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.
Mercury western:
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.
Venus eastern:
January 14th - at 47º and -4.4 magnitude, setting several hours after the Sun.

2008
Mercury eastern:
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.
Mercury western:
March 3rd - at 27º.
July 1st - at 22º.
October 22nd - at 18º, the most favourable morning apparition of the year.

2007
Mercury western:
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.
Mercury eastern:
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.
Venus western:
October 28th – at 46º and –4.4 magnitude, rising about 4 hours before the Sun.
Venus eastern:
June 9th – at 46º and –4.3 magnitude, setting just over 2 hours after the Sun.