The gravitational force between the Earth and Sun is

 When Venus is in the position of greatest eastern elongation (see diagram), it is

What is Yersinia Pestis?

Isolated atoms, such as atoms in a low-density gas cloud, only emit light at certain wavelengths.  Why?  

The two kinds of electromagnetic radiation that can easily be observed through the Earths atmosphere are

The terrestrial planets formed close to the Sun because

Venus and Earth have similar

Epicycles are used in a geocentric model to explain the

Most planets in the solar system rotate on their axes and revolve around the sun in the same direction.

The theory that the moon was formed independently elsewhere and was later captured by the earth is considered the most likely theory to be true.