Solstices and Equinoxes

Hi, all! Sorry for the late post. This post is about solstice and equinox, which we learned on 23rd Feb.

Equinoxes are the two days in the year when the day and night are exactly the same length. The vernal equinox (spring equinox) is on 20 or 21 March ( This year the vernal equinox is on 20 March which is 6 more days from now!) while the autumnal equinox is on 22 or 23 September.

Winter solstice is when the sun is above the horizon for the shortest amount of time on 21 or 22 Dec while summer solstice has the longest amount of time receiving sunlight on 20 or 21 June.

However the dates mentioned above are with respect to the northern hemisphere where we live in. For the southern hemisphere the seasons are different which means when we are having summer solstice, southern hemisphere is at winter solstice and vice versa. Quite common sense, right?

The variation of the dates is because that the revolution of earth is not exactly 365 days. It is actually 365 days 5 hours 48 minutes and 46 seconds and…… yup. So we have to include the 5 hours for calculating the exact dates.

Here is the classical picture in explaining the rationale of equinox and solstice though I think we’ve already had enough discussion on the day itself. Comparing the shaded area with the bright ones you can see the different lengths of day and night. For equinoxes we can see that when the sun directly shines on the equator, it is divided equally by day and night.

Why is it so? The earth is tilted! That causes the sun shines directly on tropic of cancer or tropic of Capricorn at different times and thus results of differences in length of the day and night.

OK. This is the most important part that we all need to understand about that lesson. Any questions please refer to the notes or leave me a comment below.

Lan Tu (8)