Depending on where you are, it’s the equinox today or tomorrow, but what does that really mean? It’s easy to define the solstices; they represent the longest day or night of the year depending on your hemisphere. On the other hand, the definition of the equinoxes is that the equator is pointed directly towards the Sun. That has much less relevance to people who live anywhere else, leading to common definitions that seem universal, but actually contain multiple errors.
Even the name equinox is inaccurate, coming as it does from the Latin for “equal night”. This leads to common references to the equinoxes being when there is an equal amount of day and night all over the world. One reason this is wrong is obvious to anyone who stops to think about it (hint: the poles), but as Space.com points out, there are also problems with the definition that require more knowledge to notice.
The first problem with the name is it assumes everything that isn’t day is night, ignoring twilight. Sunlight is scattered off particles in the upper atmosphere, which can be illuminated well before the Sun rises and after it sets. The quantity of extra light varies by location – twilight is much shorter in the tropics than in temperate regions – even if you ignore clouds. However, if you want a full 12 hours of night, you must wait a long way into winter.
Even if your definition of night includes twilight it still won’t equal the amount of daylight at the equinox because the Sun is not a point source. Before the midpoint of our local star rises and after it has set, there’s still a powerful light source covering up to a quarter of a degree of the sky shining on us all.
Once again, the Sun sets a lot more slowly at high latitudes, so no universal number covers how much extra time we get where part of the Sun is above the horizon. The minimum figure – at the equator on the equinox – is two minutes, but at the poles there are well over 24 hours of just a part of the Sun being visible.
Altitude can complicate matters too. Mountain tops stay illuminated a little longer than plains, extending the day that little bit further.
Even if you’re in a small boat on the ocean, the Earth has one more trick to play to give you some precious moments of light. That’s because the atmosphere refracts light as well as scattering it, bending it so the Sun appears to be just above the horizon when it is in fact slightly below. The day when the Sun is visible for exactly 12 hours is called the equilux, and it varies depending on your latitude.
There are plenty of online tools to help you discover the combined size of these effects where you live. These will show you how much extra daylight you get where you live beyond 12 hours. To use tools like this precisely, you also need to note which day the equinox falls on where you live. The year not being precisely 365 days long, the timing of the equinoxes and solstices move around slightly, and this year it falls at 1:04 am UTC on September 23. That means the equinox is this Friday, September 23 for Europe, Asia, Australia, and Africa, but Thursday, September 22 in the Americas.
When it comes to how much of a bonus twilight provides, however, you need to decide which twilight you care about – astronomical, nautical and civil twilights all have different definitions and lengths. As Space.com notes, the North Pole will be in civil twilight until October 8 and nautical twilight until October 25!