What if you lived on the equator?

Where you live, days may be long in summer and short in winter.

The equator is an imaginary line around the middle of Earth. What are days like there?

If you live far from the equator, you know the pattern: long, bright summer evenings and dark winter afternoons. In London, the shortest day of the year comes in December, and in our model it has less than 8 hours of daylight.

The equator is the imaginary line around Earth's middle, halfway between the poles. What would your year be like there? Would you get long summer days, short winter days, or something else? In this lab you can move a model town to the equator and watch.

Day length and noon Sun height at a latitude φ depend on the Sun's declination δ, which swings between +23.44° and −23.44° over the year. Far from the equator that swing produces strong seasons in light.

At φ = 0, the geometry behaves differently. This lab asks how much day length, noon altitude and daily sunlight change through the year on the equator, and what replaces summer and winter there.

In London in December, the Sun comes up late and goes down early.

It stays low in the sky. Winter days there are short and cold.

Start in London, at 51.5° north, on December 21, the shortest day. In the model the Sun is up for only 7.6 hours, and at noon it reaches just 15 degrees above the horizon, very low.

Six months later, on June 21, London gets 16.4 hours of daylight. That is a swing of almost 9 hours. The Met Office says the UK's winter months, December to February, are the coldest of the year, with the shortest days.

At 51.5°N the model gives 7.6 h of daylight and a noon altitude of 15.1° on day 355, and 16.4 h and 61.9° on day 172. Daily top-of-atmosphere insolation swings from 73 to 497 W/m², almost a factor of seven.

The Met Office describes the UK's winter, December to February, as the coldest months with the shortest days, and attributes the seasons to the 23.5° tilt that angles the hemisphere toward or away from the Sun.

In our pretend lab, Olly moves the town to the equator.

Now the Sun is up for 12 hours. It climbs high at noon. And it is the same in every month!

Now move to latitude 0, the equator, on the same December day. The Sun is up for 12.0 hours and climbs to about 67 degrees at noon, much higher than in London.

Try other days. On the equator the model gives 12.0 hours of daylight every day of the year. Around March 21 and September 20 the noon Sun is straight overhead, and even in June and December it is about 67 degrees up. Watch the chart: the noon Sun stays high all year.

At φ = 0, tan φ = 0, so h₀ = 90° and daylight is exactly 12.0 h on every day. The noon altitude is 90° − |δ|: 90.0° on day 80, 89.9° on day 263, and 66.6° at both solstices.

Daily insolation varies only from 397 W/m² at the solstices to 433 W/m² at the equinoxes. Notice that on June 21 the equator's 397 W/m² is less than London's 497: London's 16.4-hour day more than makes up for its lower Sun.

Earth leans as it goes around the Sun. Places near the top and bottom feel that lean a lot.

The middle of Earth hardly feels it. So its days stay the same.

Earth's leaning axis is what makes seasons, NASA explains: through the year, the north and then the south tip toward the Sun. Places far from the equator feel this a lot, getting long days and a high Sun in summer, then short days and a low Sun in winter.

The equator sits in the middle. As Earth spins, it always spends half of each turn in sunlight and half in darkness, so day and night stay about equal. And the Sun never wanders far from overhead, so it stays high all year.

Daylight is the fraction of a latitude circle lying on the sunlit side of Earth. The day–night boundary always passes through the center of Earth, so it bisects the equator whatever the tilt: h₀ = 90° exactly. Away from the equator the tilted boundary cuts latitude circles unequally, giving the long and short days of summer and winter.

The model's limits: a circular orbit, sunrise at the center of the Sun on a flat horizon, and sunlight at the top of the atmosphere. Refraction, which NOAA explains makes the real Sun appear to rise a little earlier and set a little later, is left out, and so are clouds.

Near the equator it is warm all year. The seasons are about rain.

Some months are very rainy. Others are drier.

The Met Office says that rainforest climates either side of the equator are warm all year, usually about 20 to 30 °C, with plenty of rain every month. Other tropical places have a wet season and a clear dry season.

Near the equator lies a band of low pressure called the Intertropical Convergence Zone, where winds from the north and south meet and bring frequent thunderstorms and heavy rain. The Met Office says it moves north and south through the year, following the Sun's overhead position about one to two months behind. In Australia's tropical north, the Bureau of Meteorology divides the year into a wet season and a dry season instead of four seasons.

Tropical rainforest climates have average temperatures around 20–30 °C with minor annual variation and over 60 mm of rain in every month (Met Office); monsoon and savannah climates have distinct dry seasons. The Intertropical Convergence Zone, where the trade winds converge, brings frequent thunderstorms and heavy rain and migrates with the Sun's overhead position with a lag of about 1–2 months.

The Bureau of Meteorology uses a wet season (October–April) and dry season (May–September) for northern Australia and notes Darwin's daylight varies only from about 11.5 to 12.5 hours. Temperature seasons give way to rain seasons.