The first day of spring

Now that we are in the month of March, for most of us in the northern hemisphere the worst of the winter is over, and it is only a couple of weeks until 21 March, the first day of spring.

21 March is an important date for other reasons, too. For one thing, it is used as the basis for the calculation of Easter. Early Christian decision makers in the fourth century established that Easter would be held on the first Sunday after the first full moon occurring on or after March 21, and this definition is still used today.

Traditional German Easter postcard – Image from Wikimedia Commons

March 21 is also the first day of the astrological calendar, although as a scientist I am unconvinced of astrology’s usefulness for anything other than entertainment.  In addition, in 2001 the United Nations declared 21 March as the International Day for the Elimination of Racial Discrimination (United Nations 2016).

Is March 21 the spring equinox?

There is a commonly held view that March 21 is the spring equinox and that the equinoxes are the two days in the year when all places on the Earth have exactly 12 hours of daylight and 12 hours of darkness. In fact, as I’ll explain later, this is only approximately correct. March 21 can sometimes be the date on which the spring equinox falls but its precise date varies from year to year and also depends upon where you are located.  In fact, at the equinoxes there is actually nowhere on the Earth where there are exactly 12 hours of daylight and 12 hours of darkness.

What is an equinox?

The origin of the word equinox comes from two Latin words aequus (equal) and nox (night), suggesting that at an equinox the length of the day and night are equal. However the precise astronomical definition of an equinox is slightly different.

Because the axis of the Earth is tilted rather than perpendicular to its orbit around the Sun, different parts of the Earth are closer to the Sun at different times of the year.

• At the winter solstice in December (point A in the diagram) the North Pole is tilted furthest away from the Sun than at any other time of the year, and the South Pole is tilted nearest the Sun.  In the northern hemisphere the period of darkness are longest compared with the period of daylight, and in the southern hemisphere the reverse applies.
• At the summer solstice in June (point C in the diagram) it is exactly the opposite of the winter solstice – it is the North Pole which is now tilted nearest to the Sun so  the northern hemisphere experiences the longest period of daylight.
• There are two times a year (B and D in the diagram) when the neither the North Pole nor the South Pole are tilted towards the Sun and these times are the equinoxes. At any given latitude, whether north or south of the equator, there will be the same amount of daylight.

When do the equinoxes occur?

The diagram also shows that the Earth moves in an oval, or elliptical, orbit around the Sun. This means that it has further to travel 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 therefore not exactly half a year apart: from the March equinox to the September equinox is around 186 days, whereas from the September equinox to the March equinox is only 179 days.

The tables below give the times of the two equinoxes over the next six years for three Locations: London (Greenwich Mean Time or GMT), Honolulu (GMT -10 hours) and Tokyo (GMT +9 hours).  As you can see, the northern hemisphere spring equinox can occur on March 19, 20 or 21 and the autumn equinox on Sept 22 or 23 (see note 1 below).

(TimeandDate.com 2016a)

What date of the year are there are exactly 12 hours of daylight?

If we use the normal definition of hours of daylight as being the interval between sunrise and sunset then there are actually slightly more than 12 hours of daylight at the equinox. There are two reasons for this. If sunrise were defined as when half of the Sun is above the horizon and half below, then there would be exactly 12 hours of daylight.  However, the definition of sunrise is actually the point at which the first light from the Sun’s rays reaches above the horizon, so there is actually slightly more daylight.  The diagram below shows the path of the Sun’s disk at sunrise at the equinox in London.

Similarly, at sunset the time when the half of the Sun is above the horizon and half below the horizon is 6:13 pm, shown as B in the diagram, but sunset is defined when the very last light from the Sun’s rays are above the horizon and is about a minute after this time.

In addition, when the Sun is just below the horizon, light bends, causing the Sun to appear just above the horizon. This bending of light is known as refraction and has the effect of slightly extending the hours of daylight.

Taken together, these two effects mean that there are slightly more than 12 hours of daylight at the equinox. The table below shows the dates around the equinox in London and Wellington (in the northern and southern hemispheres respectively) and it is clear to see that date on which there are exactly 12 hours of daylight and 12 hours of darkness is not 21 March.

(TimeandDate.com 2016b)

I hope you have enjoyed this post and, with apologies to my readers in the southern hemisphere, are enjoying the promise of the long summer evenings in the not too distant future.

The Science Geek

Notes

(1) The table shows that there is a pattern in that the times of the two equinoxes in a given year are just under six hours later than the previous year, unless the year is a leap year, in which case they are just under 18 hours earlier than the previous year.  Thus, the equinoxes will occur at roughly the same date and time every four years. For example the spring equinox will occur at:

• around 4 am (GMT) on March 20 in the years 2016, 2020 and 2024
• around 10 am (GMT) on March 20 in the years 2017 , 2021 and 2025
• around 4 pm (GMT) on March 20 in the years 2018 , 2022 and 2026
• around 10 pm (GMT) on March 20 in the years 2019 , 2023 and 2027

However this four-year pattern doesn’t always hold because leap years don’t always occur every four years. Century years which are not divisible by 400 e.g. 1800, 1900, 2100 are not leap years. So for example in the year 1903, where there had not been a leap year for 7 years, the equinoxes occurred relatively late. On this year the equinoxes occurred at 7:15 pm (GMT) on 21 March and 5:45 am (GMT) on 24 September. So in Tokyo Japan, which is 9 hours ahead of GMT, in 1903 they occurred at 4:15 am on 22 March and 2:45 pm on 24 September

(2) The exact day on which there is 12 hours of daylight will vary with latitude.

References

TimeandDate.com (2016) Solstices & Equinoxes for London (Surrounding 10 Years).  Available at: http://www.timeanddate.com/calendar/seasons.html?n=136 (Accessed: 5 March 2016).

TimeandDate.com (2016) London, ENG, United Kingdom — Sunrise, Sunset, and Daylength, March 2016, Available at: http://www.timeanddate.com/sun/uk/london(Accessed: 1 March 2016).

United Nations (2016) International Day for the Elimination of Racial Discrimination. Available at: http://www.un.org/en/events/racialdiscriminationday/ (Accessed: 1 March 2016).