Analemma

In astronomy, an analemma (/ˌænəˈlɛmə/; from Greek ἀνάλημμα analēmma "support") is a diagram showing the deviation of the Sun from its mean motion in the sky, as viewed from a fixed location on the Earth. Due to the Earth's axial tilt and orbital eccentricity, the Sun will not be in the same position in the sky at the same time every day. The north–south component of the analemma is the Sun's declination, and the east–west component is the equation of time. This diagram has the form of a slender figure-eight, and can often be found on globes of the Earth.

Diagrams of analemmas frequently carry marks that show the position of the Sun at various closely spaced dates throughout the year. Analemmas with date marks can be used for various practical purposes. Without date marks, they are of little use, except as decoration.

Analemmas (as they are known today) have been used in conjunction with sundials since the 18th century to convert between apparent and mean solar time. Prior to this, the term referred to any tool or method used in the construction of sundials.

It is possible to photograph the analemma by keeping a camera at a fixed location and orientation and taking multiple exposures throughout the year, always at the same clock-time (and accounting for daylight saving time, when and where applicable).

Although the term "analemma" usually refers to the Earth's solar analemma, it can be applied to other celestial bodies as well.

An analemma can be traced by plotting the position of the Sun as viewed from a fixed position on Earth at the same clock time every day for an entire year, or by plotting a graph of the Sun's declination against the equation of time. The resulting curve resembles a long, slender figure-eight with one lobe much larger than the other, known as a lemniscate of Bernoulli. This curve is commonly printed on terrestrial globes, usually in the eastern Pacific Ocean, the only large tropical region with very little land. It is possible, though challenging, to photograph the analemma, by leaving the camera in a fixed position for an entire year and snapping images on 24-hour intervals (or some multiple thereof); see section below.

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