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 Mercury 28º.
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 favorable 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 observers 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 an 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.