Autumnal Equinox 2019

John Williams

22 Sep 2019

Autumn has caught us in our summer wear.

Philip Larkin, British poet (1922-86)

The Autumnal/Fall Equinox (23 September 2019) north of the equator is mirrored south of the equator by the Spring Equinox. The Autumnal Equinox isn’t a day-long event, but rather occurs at the exact moment the sun crosses the celestial equator.

Why is it called an equinox? The word comes from the Latin aequus, meaning “equal” and nox, meaning “night.”

During the equinox, the sun crosses what we call the “celestial equator”. Imagine a line that marks the equator on Earth extending up into the sky above the equator from north to south. Earth’s two hemispheres receive the sun’s rays equally. The sun is overhead at noon as seen from the equator, so at this point, the amount of nighttime and daytime (sunlight) are roughly equal to each other. To be precise, daylight lasts about eight minutes longer than nighttime on the day of the equinox.

After the Autumnal Equinox the days shorten and nights lengthen. In astrology this is the date on which the sun enters the sign of Libra, the scales, reflecting appropriately the balanced day and night of the equinox.

photo credit: pixabay

Wind Power

Wind generation follows strong seasonal patterns. These patterns differ depending on the world region/location.

Summer heatwaves with subsequent high pressure suppress windy conditions and significantly reduce wind energy production. However, it becomes much windier in the autumn and higher wind speeds last through the winter and spring.

There is more and more wind generation being built. Therefore, it will become more common that excess power will be created from time to time. So there is great interest in developing long-term energy storage which can hold this surplus electricity for weeks or months.

The first electricity-generating wind turbine was a battery-charging machine installed in July 1887 by Scottish academic James Blyth to light his holiday home in Marykirk, Scotland.

Effects on satellites

One effect of equinoctial periods is the temporary disruption of communications satellites. For all geostationary satellites, there are a few days around the equinox when the sun goes directly behind the satellite relative to Earth (i.e. within the beam-width of the ground-station antenna) for a short period each day.

The sun’s immense power and broad radiation spectrum overload the Earth station’s reception circuits with noise and, depending on antenna size and other factors, temporarily disrupt or degrade the circuit.

The duration of those effects varies but can range from a few minutes to an hour. (For a given frequency band, a larger antenna has a narrower beam-width and hence experiences shorter duration “sun outage” windows.)

Satellites in geostationary orbit also experience difficulties maintaining power during the equinox, due to the fact that they now have to travel through Earth’s shadow and rely only on battery power.

Usually, a satellite will travel either above or below the Earth’s shadow due to its shifted axis throughout the year; during the equinox, since geostationary satellites are situated above the equator, they will be put into the shadow of the Earth for the longest period of time all year.


According to NASA, the chances of seeing aurora borealis displays increase after the Autumnal Equinox. Primarily because autumn produces a surplus of geomagnetic storms – almost twice the annual average.

At the South Pole the sun will make its first appearance for six months. However, at the North Pole it is the beginning of six months of darkness.