Field guide training: Astronomy – Part 3

Planets are celestial bodies which move in the orbit around the sun and whose mass is so big that it is in a hydrostatic balance (and therefore in a ball-shaped figure). Moreover, planets are the dominating objects in their orbits, which means the planet has cleaned its gravitation field from other objects over the time. Other celestial objects which do not fulfill all parts of the definition can be called planets from time to time as well.

• Objects in the solar system which are almost shaped like a ball and are not the dominating object in the orbit are called dwarf planets.

• Other objects of planetary mass which are not connected to celestial bodies with a greater mass such as stars are called “free floating planets”.

Planets are not able to glow, but they reflect the light of the sun. Therefore, most “stars” which can be seen right after sunset are actually planets. With binoculars or a telescope, the difference between star and planet can easily be seen: Stars can be observed as wild flickering dots while planets appears as luminous miniature-balls in the sky.

The sun is the central star of our solar system and all planets and satellites rotate around her. They also rotate around their own axis, which is caused by the gravitational interaction between the planets and the sun. Without the gravitational force of the celestial bodies the solar system would drift apart.

One of the most prominent signs of the rotation of the celestial bodies is our sunrise and sunset – technically speaking both terms are not correct, they originate from a time where humans believed that the earth was in shape of a disk. In fact, the sun remains stable and the earth is rotating around the sun and her own axis, so always one side of the earth is in the shade and the other one is illuminated by the sun. One rotation takes 24 hours.

Another misapprehension in this context is the statement that the planets rotate in circles around the sun. They rather follow an oval shaped orbit, the so-called eclipse. For one full round the earth needs 365.25 days. We define this number as a year. Other planets which are further away from the sun need a longer period of time, which means for these planets a year lasts longer than for us on the earth. The earth does not have a straight vertical axis and depending on the position and the angle of slope towards the sun, we get different seasons in the southern and northern hemisphere of the earth. For example: In December and February it is summer in Australia and winter in Europe.

From the southern hemisphere a couple of planets can be observed without the help of binoculars or a telescope: Venus, Jupiter, Mars, Mercury and Saturn. Venus can be easily seen from the earth as a bright star because it is the closest planet to the earth. Jupiter is the biggest planet in our solar system. Similar to Venus, Jupiter consists mostly from gas which is why the surface appears a bit cloudy when observed through a telescope. The second smallest planet in our solar system after Pluto is Mercury. It is very close to the sun, which makes Mercury hard to spot.

The moon is the natural satellite of the earth and approximately 383,400 km away from our planet. The light which the moon reflects does only need 1.3 seconds to reach the earth. Even though the moon appears bright when seen from the earth, only 7% of the light which illuminates the moon’s surface are actually reflected. The temperatures within the atmosphere of the moon are dependent on the phases of the moon and underlie high fluctuations (+127°C to 173°C).