Solar Zenith

Solar Zenith

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Extra information regarding Solar Zenith

Detailed write up regarding Solar Zenith by the Solcast Modelling Team

Definition

While [Azimuth] tells what direction to turn to face the Sun directly, the Zenith angle is a representation of how high the Sun is in the sky or, in other words, how much tilt the observer would need to achieve in order to be looking at the Sun directly. Specifically the solar zenith is the angle between a line pointed at the sun and a line normal to the Earth’s surface as depicted below. Its complement is the Solar Elevation Angle.

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There are two ways of defining the solar zenith angle, called apparent or actual zenith angles.

The apparent zenith angle is the angle between the direction of the sun and the vertical (zenith) point directly overhead, as observed from a specific location on the Earth's surface. It takes into account the curvature of the Earth and the observer's position, as well as atmospheric refraction effects, which cause the sun to appear slightly higher in the sky than its true geometric position.

The actual zenith angle, sometimes referred to as the geometric zenith angle, is the angle between the direction of the sun and the vertical (zenith) point directly overhead, as would be observed from a theoretical point at the Earth's surface without accounting for atmospheric effects or the observer's position. It represents the true geometric position of the sun relative to the observer's location, without any atmospheric distortion.

When requesting solar zenith, as zenith, from the Solcast API you receive the actual or geometric zenith angle.

Zenith angle and solar energy

As it is used alongside with [Azimuth] to describe the position of the Sun in the sky, the zenith angle has several applications when it comes to solar energy production. The paragraphs below will discuss some of the applications of the zenith angle in solar energy.

Zenith is the primary driver of available irradiance, affecting how irradiance changes with time of the day, location, and time of year, because the global irradiance is proportional to the cosine of zenith. This is why locations closer to the equator and summer months are sunnier, zenith angles are smaller.

Understanding the zenith angle helps determine the optimal orientation of solar panels to maximize energy capture. Solar panels are typically most efficient when they are perpendicular to the incoming sunlight. By tracking the sun's position and adjusting the tilt angle of solar panels accordingly, energy output can be optimized throughout the day.

Zenith angle information is used in shading analysis to assess the potential impact of nearby obstructions, such as buildings, trees, or terrain features, on solar panel performance. By analyzing the zenith angle of shadows cast by obstructions, solar developers can identify shading risks and optimize panel placement to minimize shading effects and maximize energy yield.

Solar zenith angle is also used to define sunrise and sunset times, as an angle of 90° indicates the sun as at the horizon.

Similarly to [Azimuth], zenith angle changes throughout the day and varies with the seasons due to the tilt of the Earth's axis. Depending on your location and therefore the zenith angles you will observe different tilts or tracking systems will perform better than others. Zenith angles are lower around the summer solstice, with the sun higher in the sky.

The figure below showcases how the zenith angle varied during the days of the winter and summer solstices as observed at the Sydney Opera House.

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