0.0 1 square kilometer is equal to one hectare, and hectares, square kilometers and square meters are all common units of area. "Square kilometer" is an area unit larger than "hectare" and is generally used to calculate a larger land area. Square meter is an international area unit and a common measurement standard in life and work.

A hectare is a metric unit of area. A piece of land with an area of one hectare is 10000 square meters, which is slightly larger than a standard football field. Square kilometer is a metric unit of area, defined as "the area of a square with a side length of 1 kilometer", and it is also a unit to measure the positive area of a banquet.

Development of astronomical units

Astronomical unit is a unit of length, which is about equal to the average distance from the earth to the sun. It is one of the commonly used astronomical units of length. The basic unit for measuring distance in astronomy, especially the distance between celestial bodies in the solar system, the average distance from the earth to the sun is 1 astronomical unit, which is about 65.438+0496 billion kilometers.

1976, the international astronomical union defined an astronomical unit as the distance between a particle with negligible mass and an object whose orbit is undisturbed and whose mass is about equal to that of a sun. When the astronomical unit was first used, its actual size was not very clear, but the distance between planets could be calculated by heliocentric geometry and the laws of planetary motion. Later, through parallax and modern radar, the actual size of astronomical units was finally found accurately.

Refer to the above content: Baidu Encyclopedia-Astronomical Units