Why does the first day of spring, summer, fall and winter differ every year and what is the greatest span of difference? It's not more than 24 hours, is it?

Why does the first day of spring, summer, fall and winter differ every year and what is the greatest span of difference? It's not more than 24 hours, is it?



SUMMER SOLSTICE = The sun is as high or as far north as it gets all year.


First day of Summer is June 21 or 22

The Sun is directly above the Tropic of Cancer (N-hemisphere)



WINTER SOLSTICE = The Sun is at its lowest, and the farthest south it gets all year.

First day of Winter is Dec 21 or 22

The Sun is directly above the Tropic of Capricorn (S-hemisphere).



EQUINOX = The Sun is directly above the equator. Rises due E and sets due W.



VERNAL EQUINOX = Spring equinox = First day of spring is March 21 or 22



AUTUMNAL EQUINOX = Fall equinox = First day of autumn is Sept 22 or 23



It varies for a few reasons.

The Earth's orbit around the Sun is not exactly circular and accordingly its orbital velocity is not constant. In addition, the Earth wobbles on its axis, an effect called precession.

Also our calender works on the basis of 24 hours in a day and 365 days in a normal year. Neither of these are true. A true day is approximately only 23 hours and 56 minutes long, and a year is about 365.25 days long.

This is why we have leap years (and seconds) to correct the errors that accumulate.

Furthermore the actual average duration in days between the annual marker points are as follows (they vary over centuries and millennia):




From any specific equinox or solstice to the next recurrence of the same:



spring equinox: 365.2424 days

summer solstice: 365.2416 days

autumn equinox: 365.2420 days

winter solstice: 365.2427 days



Since they are not all exactly the same it means they move slightly relative to each other.

This means that the lengths of the seasons oscillate.

Currently summer, autumn, winter and spring last for 93.7, 89.9, 88.8 and 92.8 days, respectively, but over the millennia, these values alter.

Sequoianoir's figures are slightly confused, but he's got the bones of it right. The figure of 23 hours and 56 minutes for a day is the length of a sidereal day, the rotation of the Earth with respect to the stars. It is not really relevant to what is being discussed.




There's no such thing as an official 'Start of Spring', as it arrives at different times in different latitudes. In Ireland, Spring starts around 15 February, but the day treated as the start of Spring is 1st February.


First Day of Spring


Spring Equinox

March (3/20/08, 3/20/09, 3/20/10, 3/20/11, 3/20/12, 3/20/13, 3/20/14, 3/20/15, 3/20/16, 3/20/17, 3/20/18, 3/20/19, 3/20/20)



•Vernal Equinox

•Spring Equinox

•Earth's Seasons

•That Shifting 1st Day of Spring

First Day of Summer

Summer Solstice - longest day of the year

June (6/20/08, 6/21/09, 6/21/10, 6/21/11, 6/20/12, 6/21/13, 6/21/14, 6/21/15, 6/20/16, 6/21/17, 6/21/18, 6/21/19, 6/20/20)



•Summer Solstice

•Summer Solstice

•Earth's Seasons - Summer Solstice

First Day of Autumn

Fall Equinox - day when night and day are of equal length


September (9/22/08, 9/22/09, 9/23/10, 9/23/11, 9/22/12, 9/22/13, 9/23/14, 9/23/15, 9/22/16, 9/22/17, 9/23/18, 9/23/19, 9/22/20)



•Autumnal Equinox

•Fall Equinox

•Earth's Seasons

•FAQ Equinoxes

First Day of Winter

Winter Solstice - shortest day of the year

December (12/21/08, 12/21/09, 12/21/10, 12/22/11, 12/21/12, 12/21/13, 12/21/14, 12/22/15, 12/21/16, 12/21/17, 12/21/18, 12/22/19, 12/21/20)


http://answers.usa.gov/cgi-bin/gsa_ict.cfg/php/enduser/std_adp.php?p_faqid=7114