Days get longer in the summer and shorter in the winter, right? We all know this. It’s a truism we all grow up with. But I was doing some astronomy reading, and realized that, in fact, that truism is completely false! Days do not get longer in the summer nor shorter in the winter.
How can this be so? Well, it’s pretty easy to prove.
The longest day of the year (in the northern hemisphere) is in the 3rd week of June – the Summer Solstice. On that day, the sun reaches its highest declination in the sky, and the length of daylight is the longest of the year. Now, if you use the usual western-style reckoning, that day is also the First Day of Summer ™. And if the first day of summer is the longest day of the year, all summer long the days must, necessarily, get shorter. And they do…
… all the way until that day in the 3rd week of December – the Winter Solstice. On that day, the sun reaches its lowest declination in the sky, and the length of daylight is the shortest of the year. Again, in western-style reckoning, that day is the First Day of Winter ™. If the first day of winter is the shortest of the year, then it follows that every day gets longer all winter long.
If you are in the southern hemisphere, you can swap December and June, but the effect is the same:
Days get shorter in the summer, and longer in the winter.