Monday the 23rd of September at 07:50 am UTC marks the point of equinox – that is when there is approximately equal day and equal night, making it the official start of Autumn.
As the Earth orbits the sun in a slightly eccentric orbit and inclined at an angle of 23.5 degrees with respect to its orbital path, it will change its orientation with respect to the sun – that is, half of the year the northern hemisphere will be pointing towards the sun and the other half of the year the southern hemisphere will be pointing towards the sun. Along this orbital path, the Earth will also reach a point of equilibrium where neither hemisphere is pointing away or towards the sun. This happens twice a year and is known as the equinox, as the sun shines directly on the Earth’s equator such that the length of day and night is approximately equal.
This year the sun appears directly over the equator at 07:50 am UTC (12:50 am PST) on Monday the 23rd of September, marking the official start of Autumn. As the Earth continues in its orbit, the nights will get longer and the days will get shorter, until the longest night on the winter solstice.
As the nights are longer and the planet’s geometry is lined up nicely for a maximum aurora zone, it makes this time of year great for viewing the Northern Lights.
Coordinated Universal Time (UTC) is the world time standard with the frame of reference being zero degrees longitude (e.g. London, United Kingdom) and with no adjustment for daylight savings.