10 Things You Didn't Know about Equinoxes and Solstices

Hopefully, anyone reading this already knew that an equinox is when the day and night are equal, and a solstice is when the day length reaches its longest or shortest throughout the year. But, did you know that there’s a lot more to equinoxes and solstices than just day length, and that there are so many misconceptions around them? In this blog we’ll take a look at 10 things you (probably) didn’t know about these events. Without further ado, let’s get started.

The vernal equinox isn’t really getting earlier - it’s our calendar that makes it appear that way. The Gregorian calendar inserts 97 leap days every 400 years, for an average of 365.2425 days per year, which is not too far off from the mean tropical year of about 365.24219 days. However, between 1900 and 2100, due to the way the calendar works, a leap day is added every four years without exceptions, making the average year 365.25 days long in this period. This is slightly longer than the tropical year, therefore the vernal equinox keeps falling earlier with respect to the calendar. Put another way, we are adding more leap days than we should.

Obviously it depends on your time zone, but in terms of UTC time, the vernal equinox in 2007 was the last one to fall on March 21 this century. Right now, it falls on March 20 (which also happens to be JetPunk Day - see this blog). In the year 2044, the vernal equinox will fall as early as March 19 for the first time. For comparison, every vernal equinox between 1900 and 1911 fell on March 21!

Again, the dates vary with the time zone you’re in. For example, if you live in America, you may have already seen March 19 equinoxes; those living in China and Japan, however, are going to get a few more March 21 equinoxes this century. But the general trend is that the vernal equinox is getting earlier due to the way the Gregorian calendar works.

(Actually, this is just one of the many reasons why the Gregorian calendar sucks. If you are interested, check out this blog where I talk about this in detail.)

Contrary to popular belief, the day length is always slightly longer than 12 hours on an equinox. This is due to two things. One of them is atmospheric refraction - the Earth’s atmosphere causes the sun to appear higher up in the sky than it actually is. This effectively makes the sun rise earlier and set later compared to no refraction. The other is the way sunrise and sunset are defined. The sun is considered to have risen as soon as its upper edge has appeared above the horizon. The same applies for sunset - the sun is deemed to have set only when its upper edge has sunk below the horizon. This again makes the sun rise earlier and set later, compared to using the sun’s center as reference.

FYI: When the day length is exactly 12 hours, it’s called an equilux.

Yes, you read that right - the shortest day of the year is actually getting longer. For many of us, this should be some good news, but it comes at a cost - the summer solstice is getting shorter at the same time. This is because of a (very) slow change in the Earth’s obliquity, which is currently decreasing: it was 23 degrees 27 minutes in the early 20th century, but by the middle of this century it will decrease to 23 degrees 26 minutes. As the Earth’s obliquity decreases, so does the difference between the maximum and minimum declination of the sun. This makes the variation in day lengths throughout the year less extreme.

Note: Sometimes you hear people talk about “the Earth’s axial precession”, and this is what they’re referring to.

This is due to the way we measure time. UTC time, which serves as the basis for civil time worldwide, is based on a combination of atomic clock time and mean solar time. However, near the winter solstice, the length of the true solar day, defined as the period from one solar noon to the next, is about half a minute longer than the mean solar day of 24 hours. This causes the earliest sunset to occur a few days (or weeks) before the winter solstice in most places. The same is true for the summer solstice: around the summer solstice, the length of the true solar day is slightly longer than 24 hours, causing the latest sunset to occur a few days after the solstice.

In astronomy, every season begins with an equinox or solstice. For example, the astronomical spring season is defined to start on the vernal equinox, and the astronomical winter season begins on the winter solstice. This is contrary to how seasons are commonly defined - spring is usually considered to begin on the first day of March, for instance.

An aside: the ancient Chinese had an unusual definition of when seasons begin. They considered each season to start on the day that’s halfway between an equinox and a solstice. For example, spring was considered to begin around February 4, and fall was considered to begin around August 7. Although most Chinese people now follow the Western definition of seasons, the Chinese one is still used in the “24 solar terms” of the modern Chinese calendar.

In most places on Earth, the sun appears to rise and set every day. However, things get a bit different on and near the Poles. There, the sun is always visible for about six months, from the vernal equinox all the way to the autumnal equinox. This is called “polar day”. And from the autumnal equinox to the next vernal equinox, the sun is always below the horizon, known as “polar night”. In other words, what would be a “day” in most places is actually a year on the Poles.

As the build-up to Christmas continues this year, have you wondered why it falls on that particular date, December 25? It’s because this date marked the winter solstice a long time ago. When Christianity entered Europe, people simply established the date of the winter solstice as the birth date of Christ, as this would facilitate their conversion to Christianity by aligning the date of Christmas with existing solstice festivities.

The reason why Christmas now occurs a few days after the winter solstice is that, when the Gregorian calendar was introduced, March 21 was considered the “standard” vernal equinox date, which unfortunately should have been March 25. This in turn affected the date of the winter solstice, which is currently around December 21.

On an equinox (vernal or autumnal), the subsolar point, where the sun is directly overhead, is on the equator. What this means is that the sun’s altitude at solar noon (the moment the sun reaches its highest point in the sky) is equal to 90 degrees minus your latitude. This should make it obvious how to determine your latitude on an equinox: simply measure the sun’s altitude above the horizon at solar noon, subtract it from 90 degrees, and you should get a number that’s within one degree of your actual latitude.

If you look at the change in day length from one day to the next near a solstice, you’ll notice that the day length hardly changes at all. This is because the change in day length, which itself is caused by the changing declination of the sun, does not occur in a linear fashion, contrary to what some people might think. In most places, the day length changes fastest around the equinoxes and slowest around the solstices.

One more thing: you may be thinking about modeling the changing day lengths with a sine wave (which would indeed have the highest slope around the equinoxes and reach stationary points near solstices if the parameters are set correctly), but unfortunately it’s even more complex than that.

If you take the dates of equinoxes and solstices throughout a year and count the number of days between them, you’ll see that they are not equally spaced. In the table below covering the period between the vernal equinoxes of 2023 and 2024, you can see this in action:

This is due to the elliptical nature of the Earth’s orbit. Near the perihelion (the point where Earth is closest to the sun), the Earth orbits the sun fastest, whereas near the aphelion (the point where Earth is farthest from the sun), the Earth’s orbital speed is the slowest.

Currently, the perihelion occurs around January 3, when the Earth is about 147 million km from the sun; the aphelion occurs around July 4, when the Earth is about 152 million km from the sun. This means that the northern hemisphere summer season is a few days longer than its winter season, as illustrated in the table above.

(Side note: Given this data, one can easily go on to calculate the eccentricity of Earth’s orbit as about 0.0167. I won’t say how, but feel free to leave a comment if you know how to do it!)

It’s been a long time since the last blog, so I really hope you enjoyed the read, and this blog managed to clear some of the misconceptions you had about equinoxes and solstices. That said, if you still have any questions about those things, feel free to leave them in the comments and I will try to answer them.

Thanks for reading!