The Autumnal Equinox 2015

  • Those in the Northern Hemisphere can look forward to scenes like this in the coming weeks. Photo credit: John Lee.

    Sunrise on Saturday 19th September with a near vertical line dividing night and day; a sign we are approaching the equinox.

    A diagram showing the overhead sun at the equator on Wednesday 23rd September 2015. Photo credit: Przemyslaw Idzkiewicz.

  • The Autumnal Equinox 2015
    19.09.2015 14:26

    For meteorologists, the autumn season is approaching its fourth week, whereas for astronomers, pagans and certain other groups, it is the Autumnal Equinox which signals the beginning of autumn.

    The Autumnal Equinox occurs here in the Northern Hemisphere this coming Wednesday, 23rd September 2015, at 0822 UTC (0922 BST). This is the exact time at which the sun passes over the equator, having spent the past six months overhead to the north. This means that there will be almost exactly 12 hours of daylight and 12 hours of darkness wherever you are in the world. It is the passage of the sun which determines seasonal changes around the world, and is responsible for the timing of hurricane seasons, monsoonal rains, alongside more subtle changes such as the leaves of deciduous trees changing colour and eventually falling. However, it is the tilt of the Earth itself as it rotates around the static sun that causes these seasonal shifts. At approximately 23.5 degrees, the tilt is such that for around six months each year, the Polar Regions experience either constant daylight or perpetual darkness.

    Whilst the coming equinox signifies a change towards cooler and darker days for the Northern Hemisphere, the sun’s southward passage means warmer and brighter days to come for those in the Southern Hemisphere.

    It’s not all doom and gloom for the UK, however, with a fairly changeable week of weather to come. After a mostly fine and warm day on Sunday, a cold front will sweep in from the west overnight and into Monday bringing a spell of rain to many areas. Some decent sunshine is expected on Tuesday albeit with a scattering of showers. A dry and fine day for much of England and Wales on Wednesday, while Scotland and Northern Ireland are likely to see rain at times. Temperatures will hover around average for September at around 15-18C, but we are likely to have a few chilly nights with temperatures locally falling into the low single figures at times.

    By: John Lee