As we come upon the Summer Solstice, people often say it’s the “longest day of the year.”  Of course, that is not really true, each day is 24 hours whether on the Summer Solstice or the Winter Solstice. What is meant is that the Summer Solstice is the day with the longest amount of daylight in the northern hemisphere.

While it is true that each day is 24 hours, our 24 hour days are getting gradually longer because the Earth’s rotation is very gradually slowing.  The slowing is occurring because the Moon’s gravity causes oceans to rise and fall – we know this action as tides.  One effect of these tides is it creates a bit of friction between the tides and the turning of the Earth causing the rotation to slow.

How much rotation slowing is occurring?  Over the past 2740 years the Earth’s rotation has lost about six hours.  To put it in perspective, a 24 hour day lengthens by nearly 2 milliseconds per century on average.  How long to add a whole second to our days? About 50,000 years? How long to add an entire hour so that we have 25 hour days? About 180 million years.