We all probably think of December has having of the shortest days of the year, since the sun is with us for the least amount of time during the day (curse you, 4 p.m. sunset!). But, did you know that December actually has the longest days of the year? It's true. It's also true,. however, that for those of us living in the Northern Hemisphere, we have the least amount of hours of daylight during this month — so how could it be possible for it to have the longest day of the year, too? MinutePhysics on YouTube explains it all in their latest video, so let's take a look.
According to MinutePhysics, the reason that we have the least amount of sunlight during this time of the year is due to the tilt of Earth's axis, which is pointing our planet away from the Sun. However, how much actual time is in our day is not determined by sunlight and actually changes throughout the year. Yes, you read that correctly — the amount of time that each day has is not stable. This is because a Solar Day is not what we typically use when we talk about the length of days, which is equivalent to the amount of time it takes for a line of longitude to rotate back to face the sun again. A Solar Day starts from the Sun's highest point on any given day, instead of at midnight as we traditionally think of it.
If I asked you how many degrees of rotation it takes for one of these points to complete a full rotation, what would you say? If you answered 360 degrees, you'd be wrong. Because the Earth doesn't idly sit in space roating, but also makes revolutions around the sun, this means that it actually takes 361 degrees to complete a full Solar Day. To explain this a little more clearly, The Polaris Project at Iowa State University also emphasizes that the Earth has to travel about 1/365th around the Sun in one day, which also creates this day length discrepancy. "The Earth takes about 1/365 of a day, or about four minutes, more to get into the same position with respect to the sun after it reaches the same position with respect to the stars," they say on their website.
Since Earth doesn't have a completely straight axis or make a perfectly circular rotation around the Sun, the whole thing gets a little more complicated. Our planet actually makes an elliptically shaped rotation around the Sun, which causes seemingly static elements like time to become much more mutable.
The speed of Earth's rotation is also not stagnant and changes depending on how close it is to the Sun — it moves faster the closer to the Sun it is. Thus, how far and fast the Earth is moving changes depending on its proximity to the Sun, which causes the amount of time in a day to change in length. When our planet is pretty close to the Sun, our day is lengthened by about eight seconds. Also, because of the tilt of the axis, how much the lines of longitude on Earth are pointing towards or away from the Sun changes, which can cause the Earth to have to rotate farther to cover the same distance during the warmer months.
As you might expect, how close the Earth is to the Sun also affects how much time is measured in a Solar Day: The Earth being pointed towards it causes the day to lengthen by approximately 21 seconds. Believe it or not, the Earth is actually pointed towards the Sun during the Winter in the Northern Hemisphere, rather than in the Summer, which makes this time of the year the longest, technically speaking. These two effects do add up over time and cause the day to become about thirty seconds longer at one point during the year, which happens to be today!
To learn more about how our geologic history is also contributing to this discrepancy in day length, watch the full video below.