Tuesday 20 August 2019

Explainer: Why the summer solstice is the longest day of the year

The Summer Solstice explained (Click to view full graphic)
The Summer Solstice explained (Click to view full graphic)

Lorraine Hanlon

Most of us probably never think of ourselves as being passengers on-board a spinning planet that whips around on its axis of rotation once every 24 hours, or that travels 970 million kilometres on its 365-day journey around the sun.

If the earth's axis of rotation was perpendicular to the sun-earth line, we would have no solstices and no strong seasonal effects. However, the 23.5o tilt of the earth's axis gives rise to different orientations of earth relative to the sun during the year, giving us both.

At the summer solstice, the northern hemisphere is tipped most directly towards the sun, leading to more concentrated sunlight and shorter shadows at noon. We observe the sun as reaching its maximum height in the sky and the day of the solstice has the greatest number of daylight hours in the year.

On the same day in the southern hemisphere, which is tipped away from the sun, the sun's rays at noon are less concentrated and shadows are longer - it is their mid-winter. The situation in the two hemispheres is reversed at the winter solstice.

The exact moment of the summer solstice, which occurs between June 20 and June 22, varies from year to year because of the influence of other planets on earth's orbit and the slight wobble of its axis. This year, the summer solstice is on June 21. In Dublin, sunrise is at 04:57 and sunset at 21.57, giving just over 17 hours of daylight.

Be glad we don't live on Venus, where the very small axial tilt of 3o provides no seasonal respite from the 462oC temperature caused by its proximity to the sun and its runaway greenhouse effect.

Professor Lorraine Hanlon, is a professor at the School of Physics, University College Dublin

READ MORE: Summer solstice 2019: A time for change in the garden


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