Solstice, Day Length, and Latitude

Willow walking in the low-angle winter light

Chickens lay fewer eggs in the winter and more in the spring and summer.  The Asian jungle fowl from which chickens originated did not lay all year but only in the spring with the express purpose of raising young like most or all other birds.  (This explains why we associate Easter, celebrated on the first Sunday after the first full moon after the spring equinox, with eggs!)

Here at the darkest part of the year, I am averaging an egg or two per day.  Penelope, the Easter Egger who infamously hid her eggs in the trash pile, and Rosie, the Welsummer hen, are the only two  of my nine pullets currently laying, often taking turns by the day.  Hens’ laying cycle is affected by day length; poultry experts say that a hen needs at least 14 hours of daylight to lay and usually recommend supplementing with artificial light in the morning to attain those 14 hours.  I thought this must be affecting most of my girls, however Gail Damerow says:

Pullets, or young chickens, will generally lay through the first winter. After that, hens generally stop laying as day length decreases in the fall, unless provided with lighting to augment natural light for a total of 15 hours per day. So on average — again, depending on the breed, feed, and management — a hen lays about 240-250 eggs per year; pretty good would be 300 per year. After each year of laying, the number of eggs decreases and the size increases.

Being pullets and not hens, I guess more of my girls should be laying.  Other factors like feed or breed might be to blame.  I’m not too worried – I expect a landslide of eggs this spring (or else!) – but this 14-hour clause got me to thinking.  How many 14-plus-hour days are there in my area anyway, or anywhere else?  I remembered that Eliot Coleman discussed the relationship between day length and latitude in the introduction to his Four-Season Harvest.  Day length also affects the growing season for plants, and Eliot explained that his Maine farm on the 44th parallel experienced the same day length as Cannes, France.  They shared, as he termed it, a “sun line.”

On the winter solstice, December 22, I walked back from my chicken coop on the 38th parallel north with two eggs in the evening dusk of a 9-hour and 29-minute day, the same length here as in the Andalusia region of Spain, the Greek city of Athens, and the controversial border between North and South Korea, each of us receiving our 9 hours and 29 minutes in turn as the sun passed overhead along our sun line.  Meanwhile, as the northern axis of the earth tilted away from the sun, the Antarctic had a full 24 hours of sun, Eliot Coleman had 8 hours and 53 minutes of sun, and the Artic had no sun at all – 24 hours of darkness.

On that day, the winter solstice for those of us in the northern hemisphere and summer solstice for those in the south, the sun passed directly over the Tropic of Capricorn in the southern hemisphere, making that sun line the latitude of overhead sun or the subsolar point where the sun’s rays shone from above at a 90° angle.  On the equinoxes, the axis of the Earth is sideways to the sun, the equator is directly beneath the sun, and all regions of the Earth receive an equal 12 hours of daylight.  On our summer solstice, the Tropic of Cancer is subsolar and the 38th parallel will receive our maximum of 14 hours and 50 minutes of sun in a single day.

Playing with the sunrise and sunset calculator here, I determined that we 38th-parallelers will get 14 hours or more of sun from approximately May 11th until August 8th each year, 90 days.  Theoretically, I will need to add artificial light to keep my hens laying both before and after these dates.  I don’t expect to do this, but I had a fantastic time figuring all of this out.  I found this YouTube video, from which I have borrowed the images above (because the original video I found, which was longer, explained that it is public domain), to be very helpful in understanding the relationship between day length and latitude.  I also enjoyed this video which explains other aspects of seasons and annual cycles in greater detail.

Shared with:

The Self Sufficient HomeAcre


11 thoughts on “Solstice, Day Length, and Latitude

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  3. I also noticed a jump in egg production about this same time; despite having a 60 watt bulb in since early Dec, the egg production decreased until only about 6 eggs a day ( down from 22 in late summer). I thought the 60 watt incandescent wasn’t enough, so I replaced it with a 200 watt. SOme increase in production, to about 12 eggs a day. The egg number continues to increase. Would have thought all the hens would have come back online all together . . .

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