The Earth and all the other planets orbit the Sun. The closer a planet is to the Sun, the faster it revolves around it. As we observe the planets from the Earth, which is itself continuously moving, the movements of the planets in relation to the stars are often puzzling.
Venus is one of the most frequently observed objects in the sky. Long before the invention of the telescope, people were interested in the movements of the planets as a basis for measuring time, for determining harvest times or annual feast days. The Mayas, a south American people, even had a Venus calendar with 584 days. The orbit of Venus lies between the Sun and the Earth. The planet is only visible in certain positions relative to the Earth, being hidden during both upper and lower conjunctions.
A conjunction is when several objects, in this case Earth, Venus and the Sun, are on the same meridian. When Venus lies in eastern elongation it is visible after sunset as the evening star. When it lies in western elongation it is seen as the morning star before sunrise.
The visibility of a planet naturally depends to a great extent on meteorological conditions. High humidity or fog makes observation impossible. Light pollution also limits visibility. Similar to the Moon of the Earth, Venus is seen to have phases of waxing and waning. These are quite clearly visible. The largest is the thin Venus crescent, which is seen when Venus is closest to the Earth.
The visibility of individual planets is noted in special calendars made on the basis of many years of experience. Times may vary by a few days depending on location and weather. Observations of Venus by telescope do not reveal surface details, as the opaque atmosphere shrouds the planet completely.
Mercury is only ever visible for short periods from the Earth. It never reaches a distance from the Sun sufficient to allow it to be visible for an entire night. Mercury is completely invisible when it lies behind the Sun relative to the Earth. This position is known as the upper conjunction with the Sun. It is also invisible when it passes exactly between the Sun and the Earth. This is known as a lower conjunction.
Mercury is visible in the evening when it is on the left or eastern side of the Sun. This is the so-called greatest eastern elongation from the Sun. It is visible in the morning when on the right or western side of the Sun. This is known as the greatest western elongation from the Sun.
Mercury can rise or set up to two hours before or after the Sun. However, it is not always visible as it is also necessary for it to be significantly further north in the zodiac than the Sun. The period from the beginning of August to the beginning of December is suitable for observation in the western elongation, the period from mid January to mid-May for observation in the eastern elongation.
Mercury appears very small when viewed through a telescope. No surface details are visible, but phases similar to those of the Moon of Earth can be observed. As with the Moon, new Mercury (lower conjunction) and full Mercury (upper conjunction) can be observed. The planet is easiest to find if one knows the exact times it is near to the waxing or waning crescent of the Moon.
In the northern hemisphere, Mercury was most clearly visible on the 3.3.99 and the 15.2.2000.
