Before telescopes were invented, people knew only five planets, the ones we call Mercury, Venus, Mars, Jupiter and Saturn. The names were given by the Romans to honor their gods. You can see any of these five with your unaided eye, but this article is to help you to find and recognize Mercury and Venus.
Mercury is the most elusive of the planets we can see with the unaided eye, because it is always close to the Sun. This means that if it's visible, it will be low on the horizon. So if you live in a built-up area, you need to find a spot with a clear horizon in order to look for it.
The best seasons for viewing Mercury are spring and fall. In the spring it's low on the horizon shortly after sunset and in the fall it's low on the horizon before sunrise. You can get predictions of its visibility from websites such as the excellent one run by Sky & Telescope or look for it on a sky chart at Heavens-above.com.
The best way to find Mercury in the evening sky is to use binoculars. Start scanning the western horizon with binoculars shortly after sunset. Never try this before the Sun has set - you donít want to risk looking accidentally at the Sun, which can permanently damage your eyesight.
Through binoculars, Mercury looks like a bright, slightly orange star. Once youíve found it with binoculars, you can continue to follow it with the unaided eye as the sky darkens, until it finally sinks too low.
The planet itself is like a larger version of our Moon, with a rocky surface covered in craters caused by impacting meteors. A space probe called Messenger is currently investigating it in detail.
One planet youíll have no difficulty spotting is Venus, named after the goddess of love, which is by far the brightest of the planets. In fact, youíve probably seen it many times without realizing it. Itís the brilliant object popularly termed the evening or morning star, shining in the twilight for some hours after sunset or before sunrise.
Because of its brightness itís also frequently mistaken for a hovering UFO. One famous person who made this mistake was Jimmy Carter, at that time still governor of Georgia, but later to become President of the USA.
Venus appears so bright for two reasons. Firstly, it comes closer to us than any other planet, and secondly itís covered in thick clouds that reflect most of the sunlight hitting them.
As Venus goes around the Sun, it shows phases like those of the Moon. Venus is actually at its brightest when itís a crescent, shortly before or after passing between us and the Sun. You can see the crescent shape through binoculars (if held steadily) and small telescopes, but because of the clouds that envelop the planet you wonít see anything else. Space probes have shown us that the surface of Venus is far from heavenly - itís a volcanic desert with roastingly hot temperatures.
Mercury and Venus are sometimes called the inferior planets. This doesn't reflect any earthly snobbery. They simply orbit closer to the Sun than Earth does. All of the other planets are superior planets because their orbits are farther away from the Sun than ours.
Do planets twinkle?
Itís often said that planets donít twinkle like stars, but thatís not strictly true. Stars twinkle because their light is broken up by air currents as it passes through the Earthís atmosphere. Planets, on the other hand, arenít points of light like stars, but have small disks. This reduces the amount of twinkling, but doesn't always get rid of it entirely. When a bright planet is low on the horizon it does twinkle slightly, but much less than a star would.