2.1.3 Present-day insolation at the top of the atmosphere

The instantaneous insolation, defined as the energy received per unit time and unit surface on a horizontal plane at the top of the atmosphere (or on a horizontal plane at the Earth's surface, if we neglect the influence of the atmosphere) depends on the geographical position on Earth as well as on the position of the Earth relative to the Sun.
The influence of those factors is described in the following sections.

• 2.1.3.1 Earth's orbit around the Sun
• 2.1.3.2 Computation of the zenith angle
• 2.1.3.3 Daily insolation at the top of the atmosphere