The Sun: late rising, twilight and a lady computer

Due to refraction of light by the atmosphere, the Sun appears to rise before it crosses the horizon.

A Dutch explorer, Gerrit de Veer, first recorded the phenomenon on an expedition to find a northeast passage to China. He reported that during the polar winter the Sun was visible two weeks before calculations suggested it should return.

Sunset and sunrise are defined as the times at which the upper limb of the Sun contacts with a horizon of 0°. The non-uniform density of the atmosphere causes the Sun’s rays to follow curved paths so that the Sun’s apparent position differs from its true location.

Observations made from Edmonton, Alberta, suggest an average refraction of 0°.7, though the size of the effect depends on a number of factors, including the temperature of the atmosphere. The phenomenon is sometimes referred to as the Novaya Zemlya effect and, in extreme cases, for example on 10 January 1991, sunrise can appear to occur as much as 12 minutes before the Sun actually crosses the horizon.

Types of twilight

Once the Sun has passed below the horizon, the atmosphere continues to be illuminated by the scattering of light in the atmosphere. The twilight period is divided into three categories: civil twilight is the interval between sunset and the time the Sun is 6° below the horizon; nautical twilight occurs when the Sun is between 6° and 12° below the horizon; and astronomical twilight is the period when the Sun is between 12° and 18° below the horizon.

As the sky remains illuminated during civil and nautical twilight, astronomers often use red lights that maintain the dark adaptation of the eyes once astronomical twilight begins.

A pioneering female astronomer

Annie Maunder made significant contributions to the study of the Sun. Ineligible for a degree, in 1889 Maunder was the highest ranked mathematician in her year at Girton College Cambridge, and went on to work as a “lady computer” at the Royal Observatory, Greenwich. There, she devoted her time to photographing the Sun and tracking sunspot activity, contributing to the development of the “butterfly diagram” of sunspot movements.

Following her marriage to Walter Maunder, she was forced to curtail her research due to expectations on married women at the time. Undeterred, she obtained a grant to buy her own camera and took part in several overseas expeditions, photographing eclipses and the solar corona. She published her research and co-authored a popular book on astronomy with her husband, though Walter observed that the text was “almost wholly the work of my wife”.