Are there planets the size of the earth?

Earth - moon

Earth and moon in size comparison.

(Photos: NASA)

We humans on earth look at the neighboring planets in the solar system and the stars in the night sky with the knowledge that these will not be accessible to us humans, at least in the next few years. But science tries intensively to research these celestial bodies as comprehensively as possible from a great distance and to describe them. We from the Linz Astronomical Community try this here in a humble way with our planetary path.

But how would an extraterrestrial visitor describe our own planet?
Maybe something like this:

Starting from the sun, the earth is the third planet. Even from a great distance it can be seen as a blue disk against the black background of the universe. For the time being, one suspects a double planet, because the planet's only natural companion, the moon, is almost a third the size of the blue planet. There is no significant atmosphere surrounding the moon, so a solid surface strewn with craters can be seen. In the earth's atmosphere, on the other hand, there are clouds of water vapor, which do not allow a direct view of the surface of the planet everywhere.

Almost a third of the earth's surface consists of solid ground, and more than two thirds are covered with water. 98% of the water on earth is salt water, the rest is fresh water, around two thirds of which is bound to the poles as ice in a solid form. The earth is the only planet in the solar system on which water is permanently present in all three aggregate states (as ice, liquid water and as water vapor). This water is the basis for the existence of a biosphere with the most diverse forms of life that cannot be found anywhere else in the solar system. The atmosphere, which consists of approx. 78% nitrogen and approx. 21% oxygen, also contributes to this uniqueness, as it ensures relatively low temperature fluctuations compared to other planets: The temperature on the earth's surface is in the range of -89 ° C to + 58 ° C, the mean value is + 15 ° C. Without a protective atmosphere, the average temperature on earth would only be around -18 ° C.

The earth is about 4.6 billion years old. The first simple life forms developed 3.9 to 3.5 billion years ago, the first multicellular life forms can be detected 2.1 billion years ago.

The earth from the human perspective: A description of numbers and quantities that we have derived from the earth.

We have derived the basic units for our time measurement - the day and the year - from the duration of one full revolution of the earth around its axis and the duration of one complete orbit around the sun. The time it takes for the moon to orbit the earth is called a month. From the forty millionth part of the equatorial circumference of the earth, we derived the most common unit for length measurements, the meter measure. We have defined the mean distance from the earth to the sun, which we call the astronomical unit, as a measure of length for interplanetary distances. The freezing and boiling points of water are the fixed points on our temperature scale (degrees Celsius).

These examples are incomplete, but the application of these numbers and orders of magnitude have become an important part of our lives and enable us to compare them with - in our case - other celestial bodies:

earthAbsolutelyRelativePlanet path
Distance from the sun150 million km1.0 x earth75 m
diameter12,756 km1.0 x earth6.4 mm
Dimensions6.0x1024 kg1.0 x earth0.75 g
Severity acc.9.8 m / s²1.0 x earth
Rotation time23H 56m 41s1.0 x earth
Orbital time365.25 days1.0 x earth
Orbit speed30 km / s1.0 x earth
Light transit time8m 18s

Seen from the North Pole, the earth rotates counterclockwise on its axis. The orbit around the sun is also counterclockwise. The earth orbits the sun at a speed of almost 30 kilometers per second or around 108,000 kilometers per hour. It rotates once on its own axis in just under 24 hours. The speed of the earth's rotation at the equator is 1667 km / h or 463 m / s. The self-rotation is so fast that the centrifugal force creates a bulge in the equatorial area and flattening at the poles. The diameter of the earth at the equator is around 43 km larger than from pole to pole. This special shape of the earth, which deviates from the spherical shape of the earth, means that not the summit of Mount Everest (8848 m), but the summit of the 6268 m high Chimborazo in Ecuador is furthest away from the center of the earth.

The axis of rotation of the earth is inclined by 23.5 ° to the plane of the orbit. In the course of one orbit around the sun (one year), this causes the sun to irradiate the northern or southern hemisphere with varying degrees of intensity and thus leads to the development of the seasons.

The earth is surrounded by a two-pole magnetic field, which almost completely prevents the particles of the solar wind from hitting the earth. The position of the poles of the magnetic field shifts by a few kilometers every year. The axis of the magnetic field currently deviates by around 11.5 ° from the Earth's axis of rotation.

The moon

The moon has a diameter of 3476 km, which means that the earth is only 3.7 times larger than its satellite. That is the smallest difference in size between a planet and the corresponding moon in our solar system. Earth and moon move around a common center of gravity, the barycentre, which, however, is still inside the earth, about 1700 km below the earth's surface, since the mass ratio between earth and moon is about 81: 1.

The moon revolves around the earth once in 27.3 days. In the same time it turns once around its own axis, so that it always faces the same side of the earth. The moon stabilizes the position of the earth's axis and thus keeps the position of the climatic zones and the living conditions on earth constant.

The emergence of ebb and flow on earth is essentially caused by the gravitational pull of the moon. The surface to be conquered, both on land and in water, is raised almost half a meter by the tides. The size of the tidal range (difference between ebb and flow) on the coasts of the earth is largely determined by the flow movements of the water. The accumulation of tidal waves on the coasts has a braking effect on the earth's rotation, so that the day length increases by around 20 microseconds per year.

moonAbsolutelyRelativePlanet path
Distance to earth384,400 km19 cm
diameter3,476 km0.27 x earth1.75 mm
Dimensions7.4x1022 kg0.012 x earth0.009 g
Severity acc.1.6 m / s²0.17 x earth
Rotation time27d 7H 43m27.3 x earth
Orbital time27.3 days
Orbit speed1.0 km / s0.03 x earth


Aristarchus of Samos tried to determine the distance of the moon to the earth as early as the 2nd century BC. He determined a distance of 19 earth radii (actual value: 60 earth radii) and was already correct in terms of magnitude.

Described after the invention of the telescope Galileo Galilei 1609 the lunar surface and its craters in detail for the first time. However, his observations sparked violent contradictions, since the natural philosophers were in the tradition of the Aristotle firmly convinced that celestial bodies could only be perfect (i.e. smooth) spheres.

In 1666 deliberations resulted Sir Isaac Newton to the realization that the force that makes an apple fall to the earth must be the same force that keeps the moon on its orbit around the earth, and so led him to the formulation of a universally valid law of gravitation.

January 4th, 1959 - only fifteen months after the launch of the first artificial earth satellite Sputnik 1 - flew the Soviet Lunik 1 as the first space probe ever to pass the moon. Struck on September 13, 1959 Lunik 2 as the first man-made object to be targeted on the surface of the moon. Sent about a month later Lunik 3 the first pictures from the back of the moon, always facing away from the earth, to the ground station. The first soft landing of a space probe on the moon succeeded with Luna 9 on February 3, 1966 also the Soviet Union.

The US astronaut stepped on July 21, 1969 at 3:56 a.m.CET Neil Armstrong as the first person the moon and thus for the first time a celestial body. A total of six moon landings were carried out between 1969 and 1972 as part of NASA's Apollo program. This means that 12 people have set foot on the moon to date.


Even binoculars can reveal individual craters on the moon. A small telescope shows countless craters and mountain ranges on the lunar surface. For observing surface details on the moon, the nights around the crescent moon are best, when mountains and craters cast long shadows and so emerge vividly. The time around the full moon is less suitable for such observations.

The moon, taken with the telescope of the Kepler observatory in Linz.

(Photo: Herbert Raab / LAG)