Do The Tropics Get Two Longest Days Because The Sun Passes Overhead Twice A Year?

If you live at the equator the Sun passes overhead twice in the year, at the equinoxes. In late June the tilt of the Earth means the Sun doesn’t get higher than 67 degrees above the horizon – which is still pretty high – and around Christmas it is the same height, but to the south, not the north. So when someone asked IFLScience whether this meant there were two longest days each year in the tropics, we thought it sounded like it might be right.

The first thing we found when we tried to check is that the answers don’t seem to be that easily available. Some experimental efforts with a search engine didn’t come up with any examples of other science sites that have answered the question; no doubt some have, and we weren’t using the right search terms. Still, it does seem anyone casually googling might not get a helpful answer. We knew if we asked ChatGPT we’d get a confident answer, but quite likely a wrong one.

It’s also not easy to answer the question by thinking about the geometry of the situation. It makes sense that when the Sun goes from due East to due West and passes directly overhead that it would take longer than when it makes a shorter journey, but that’s not necessarily right. After all, the longest days are in summer above the Arctic Circle (or below the Antarctic Circle) and the Sun never actually gets very high for an observer near the poles.

The population of the tropics is large and growing fast, which raised the further question: was this something everyone living there would know? Would a billion people laugh at us for not knowing the answer? Still, embarrassing as that might be, better to investigate than to not try for fear of looking stupid.

Consequently, we took a different approach and turned to the very useful timeanddate.com site. We half expected they’d have written their own explainer on the question, but if they have, we can’t find it. Instead, we tried using their function that tells you sunrise and sunset times for a variety of cities.

Tropical cities provide the answers

Singapore is one of the closest large cities to the equator (1 degree 17 minutes north) so this seemed a good place to start. Timeanddate.com reveals Singapore has just one longest day, and it’s the same as for everyone else in the northern hemisphere – June 20 or 21 depending on the year.

Unlike at higher latitudes, you probably won’t notice the difference between the seasons much. The longest day lasts 12 hours, 11 minutes, and 46 seconds.

You might expect that the winter solstice would be the opposite – 11 hours, 48 minutes, and 14 seconds – to give an exact annual average of 12 hours, but that’s wrong. Even the shortest day in late December has just over 12 hours and 3 minutes. The reason is that while the middle of the Sun spends exactly 12 hours above the horizon on average through the year, it’s still daylight while some part of the Sun is up. It takes a few minutes in the morning between the leading edge rising and the center of the Sun making its appearance; likewise we get a few extra minutes at sundown after the midpoint has set. Consequently, the day is an average of a little more than 12 hours everywhere, tropics included.

Nairobi is about as far south of the equator as Singapore is north, and unsurprisingly, it’s pretty much a mirror image. The shortest day (in June) has 12 hours, 2 minutes, and 51 seconds of daylight. On December 21, however you get a whole extra 9 minutes and 8 seconds – spend it wisely.

In March and September, both cities are an exact match – like everyone else they’re getting about 12 hours and 6 minutes of daylight at the equinox.

The tropics are defined as the area between the Tropic of Cancer in the north and the Tropic of Capricorn in the south, each of which lies 23 degrees 26 minutes from the equator.

That’s quite a bit of territory, and closer to the boundary lines the days vary a lot more than right near the equator.

Rio de Janeiro lies just barely inside the Tropic of Capricorn. Consequently, the Sun is directly overhead at noon (give or take a little for time zones) 12 days before, and 12 days after the summer solstice. That doesn’t make those the longest days, however. The day length will peak on December 21, and unlike in Singapore, you’ll notice it, with more than an hour of extra sunlight compared to the average, and two and a half more than in midwinter.

Why aren’t days longest when the Sun is overhead?

We’ve established that the even in the tropics the days are longest at the solstice, even though the Sun is lower in the sky that at some other points in the year, but why?

This represents yet another proof that the Earth isn’t flat, although anyone who has ignored all the more obvious ones isn’t going to be convinced by this. On a flat Earth, circled by a Sun, we would indeed expect to have longer daylight when the Sun passes straight overhead, and therefore travels a longer arc. However, on a spinning ball things are different. In summer, an entire hemisphere is turned towards the Sun, and the closer it is to the solstice, the greater that orientation is, keeping the Sun above the horizon for longer.

How obvious is this?

As noted above, we wondered whether this was a question that could easily be answered by anyone who had lived in the tropics – which is a large and growing proportion of the global population.

However, a look at the examples we gave shows that for much of the tropics the question isn’t something you could answer just by looking out your window regularly. Close to the equator, like our example cities Singapore and Nairobi, the difference in the length of the days is so small it would be hard to notice that it changes, let along by how much. “Seasons” refers to the wet and dry, not how long the Sun is up.

On the other hand, for a city at the edge of the tropics the answer would also not be obvious. In Rio, the Sun passes directly overhead so close either side of the summer solstice that the days are effectively indistinguishable then from the solstice itself – the length only varies by two minutes between when the Sun is highest and the solstice itself.

There are places in the tropics where the answer to the original question would be a lot more obvious. Take Bangkok for example. At 14 degrees north, the Sun passes directly overhead in late April and mid-August. At those times of year the days last about 12 hours and 35 minutes. By comparison, in at the summer solstice, the year lasts 12 hours and 56 minutes. Those 20 minutes are enough to make the difference noticeable.

So yes, there probably are a lot of people who think we’re stupid for even asking, but nothing like the entire population of the tropics.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.