In 2021 September 22 is the date of the September equinox and can be the first day of autumn (or the first day of spring if you happen to’re one in all my readers in the Southern Hemisphere). In this put up I’ll speak about the equinoxes and discus the generally held view that they’re the two days in the year when all locations on Earth have precisely 12 hours of daylight and 12 dead nights.
What is an equinox?
The origin of the phrase equinox comes from two Latin phrases aequus (equal) and nox (evening). This definition means that at an equinox the size of the day and evening are equal. However, the exact astronomical definition of an equinox is barely totally different.
The diagram above exhibits the Earth going round the Sun in its orbit.
- At the December solstice (level A in the diagram) the North Pole is tilted additional away from the Sun than at some other time of the year, and the South Pole is tilted nearest the Sun. In the Northern Hemisphere the interval of darkness is longest in contrast with the interval of daylight, and in the Southern Hemisphere the reverse applies.
- At the June solstice (level C) it’s precisely the reverse of the winter solstice – the North Pole is now tilted nearest to the Sun. So, the Northern Hemisphere experiences the longest interval of daylight.
- There are two occasions a year (B and D in the diagram) when the neither the North Pole nor the South Pole are tilted in the direction of the Sun and these occasions are the equinoxes. If we take two locations with the identical latitude, one in all which is north of the equator and the different one south of the equator and roughly the identical longitude (for instance Tokyo and Adelaide, South Australia) they’ll each have the identical quantity of daylight at the equinox.
On what date do the equinoxes happen?
The Earth strikes in an elliptical orbit round the Sun. This implies that it has additional to journey in its orbit between the March equinox and the September equinox than in the return leg of its journey from September to March. The two equinoxes are subsequently not precisely half a year aside. The time between the March equinox and the following September equinox is round 186 days, whereas the time between the September equinox and the following March equinox is barely 179 days.
For 2021/22 the dates and time of the equinoxes are
- 20 March 2021 at 09:37
- 22 September 2021 at 19:41
- 20 March 2022 at 15:33
- 23 September 2022 01:04
All occasions are in GMT (which is extra accurately often called UTC). So, relying in your time zone, the equinoxes will happen at a special time and even a special date.
Are there 12 hours of daylight at an equinox?
When we reply this query, we’d like to consider what imply by the time period: ‘hours of’daylight’?
Do we contemplate twilight, the time simply after dawn or simply earlier than sundown when it’s not utterly darkish, to be daylight?
Or will we contemplate daylight as being the time when the Sun is above the horizon?
If we use the definition of ‘daylight’ as being the interval between dawn and sundown, then for 2 causes there are literally barely greater than 12 hours of daylight at the equinox all over the place in the world.
- Firstly, the definition of dawn is when the first mild from the Sun’s rays reaches above the horizon, not when the centre of the Sun is above the horizon. The diagram beneath exhibits the path of the Sun’s disc round dawn at the equinox in London. In the early morning, the time when the half of the Sun is above the horizon and half beneath the horizon is 6:48 am, proven as B in the diagram, however dawn is a few minute earlier than this time.
Similarly, in the early night the time when half of the Sun is above the horizon and half beneath the horizon is 6:56 pm, B in the diagram, however sundown – when the final mild from the Sun’s rays is above the horizon – is about a minute after this time.
- Secondly, when the Sun is just under the horizon the Earth’s environment bends the Sun’s rays, inflicting them to seem simply above the horizon. This bending of sunshine is called refraction and has the impact of barely extending the hours of daylight.
Taken collectively, these two results imply that there are barely greater than 12 hours of daylight at the equinox. The date on which there (nearly) precisely 12 hours of daylight and 12 dead nights is referred to as the equilux (though this time period shouldn’t be extensively used and is so rejected by my spelling checker 😉 ). In London the equilux happens on 25 September, three days after the equinox. For different areas on the Earth it’s going to happen on totally different dates
When does Autumn start?
Although I began this put up by saying that September 22 is the first day of the Northern Hemisphere autumn, in the UK there may be an alternate definition of autumn utilized by meteorologists and extensively utilized by the public as effectively. Meteorological autumn include the months of September, October and November and so begins on September 1 and finishes on November 30. Similarly, meteorological winter is December/January/February, spring is March/April/May and summer season June/July/August. To many individuals, it makes extra sense for the center of December with its low temperatures and quick hours of daylight to be in winter than in autumn!