Latest sunrise, sunset at temperate latitudes
Photo: Peter Bowers
Saturday, January 3, 2009.
At temperate latitudes in the northern hemisphere – like those in the US and Europe – the latest sunrises of the year come around now.
That’s true even though the shortest day happened 2 weeks ago on the December solstice. Meanwhile, at temperate latitudes in the southern hemisphere – like in Australia and New Zealand – the latest sunsets comes around now. The longest day in the southern hemisphere happened 2 weeks ago, also on the December solstice, which is their summer solstice.
The exact date for the latest sunrises or sunsets varies by latitude, but the sequence is always the same. The earliest sunsets come before the winter solstice. The latest sunrises come after the winter solstice.
It’s the same in summer, as well. In summer, it’s earliest sunrise, summer solstice, latest sunset.
It happens because of the tilt of Earth on its axis as it orbits the sun. At this time of year, clock time gets ahead of true solar noon for several weeks around the solstice.
And that also causes the clock to show a later time for sunrise and sunset, so it’s really just our clocks temporarily out of synch with nature.
Although the tilt of the Earth’s axis is responsible for the earliest sunrise/sunset – solstice – latest sunrise/sunset phenomenon, the Earth’s eccentric orbit does have an impact, either accentuating or lessening the effect. The Earth is closer to the sun on the December solstice, which accentuates the effect. The sun is farther away from the sun on the June solstice, and that lessens the effect.
At middle temperate latitudes, the earliest sunrise/sunset comes about 2 weeks before the December solstice and the latest sunrise/sunset comes about 2 weeks after. But in June, the earliest sunrise/sunset comes about one week before the June solstice and the latest sunrise/sunset occurs one week after.
Actually, it has relatively little to do with the tilt of the Earth’s axis and a whole lot to do with the ellipicity of Earth’s orbit round the sun. Earth’s orbit around the sun is an ellipse with the sun located at one focus, not at the center. This causes an apparent displacement of the sun relative to Earth’s horizon. It also means that Earth moves faster in its orbit when closer to the sun and slower when further from the sun, causing an additional displacement.
J. Smyth,
The procession is always the same: the year’s earliest sunset precedes the winter solstice, and the year’s latest sunrise follows the winter solstice. One-half year later, it’s the same idea: the year’s earliest sunrise comes before the summer solstice, and the year’s latest sunset comes after the summer solstice.
It’s the tilt of the Earth’s axis that is responsible for this phenomenon. Around the solstices, the day as measured from solar noon to solar noon exceeds 24 hours in length. Around the equinoxes, the day as measured from solar noon to solar noon falls shy of 24 hours long. This would be the case even if Earth’s orbit were perfectly circular.
However, as you point out: Earth’s orbit around the sun is an ellipse with the sun located at one focus. Because the Earth is closer to the sun at the December solstice than at the June solstice, the day as measured from solar noon to solar noon is about 1/2 minute longer than 24 hours long around the December solstice, but only about 1/4 minute longer than 24 hours around the June solstice.
Click here and scroll to the bottom of the page to see the diagram illustrating the variation in the equation of time due to obliquity (tilt of Earth’s axis) and unequal motion (eccentricity of orbit).
Bruce
EarthSky Tonight feature:
Earliest sunset today, but not shortest day