What if June were winter?

In many places, June is summer. It is warm and the days are long.

But what if June were winter instead? In some places, it is!

If you live in Europe or North America, June means summer: long evenings and warm weather. December means winter.

But in Australia, the Bureau of Meteorology counts June, July and August as winter, and December, January and February as summer. So on the very same day, one half of Earth can be enjoying summer while the other half is in winter. How can that be? In this lab you can compare London and Sydney on the same dates.

Seasons are not global. The Bureau of Meteorology defines Australian winter as June to August and summer as December to February, the reverse of the Met Office's UK calendar.

This lab compares the same date at two latitudes on opposite sides of the equator, 51.5°N (London) and 33.9°S (Sydney), and asks what the declination of the Sun does to day length, noon altitude and daily sunlight in each hemisphere.

On June 21 in London, the Sun is up for a long time. It climbs high in the sky.

It is the longest day of the year there. Summer!

Start in London on June 21. In the model the Sun is up for 16.4 hours and climbs to about 62 degrees at noon. The Met Office says the June solstice, on 21 or 22 June, gives the UK its longest day and shortest night.

London's summer runs from June to the end of August, the Met Office says, with the hottest temperatures and sunniest days.

At 51.5°N on day 172 the model gives δ = 23.4°, a noon altitude of 61.9°, 16.4 h of daylight and 497 W/m² of daily insolation at the top of the atmosphere, 115 percent of the equator's March value.

The Met Office describes the June solstice as the moment the Northern Hemisphere is fully tilted toward the Sun, giving the UK its longest day, and uses meteorological seasons with summer from June to August.

Now Olly flies to Sydney, far south of the equator. It is still June 21.

The Sun is low. The day is short. It is winter there!

Keep the same day and move to Sydney, at 33.9° south. Now the model gives only 9.7 hours of daylight, and the noon Sun reaches just 33 degrees, much lower in the sky. June 21 is Sydney's shortest day.

Now change the day to December 21. London's day shrinks to 7.6 hours, while Sydney's grows to 14.3 hours, with the noon Sun almost 80 degrees high. The seasons have swapped.

At 33.9°S on day 172 the model gives a noon altitude of 90° − |φ − δ| = 32.7°, 9.7 h of daylight and 193 W/m², 45 percent of the equinox-equator reference.

On day 355 (δ = −23.4°) the pattern reverses: London 15.1°, 7.6 h and 73 W/m²; Sydney 79.5°, 14.3 h and 495 W/m². The hemispheres trade places across the year.

Earth leans over a little. In June, the top half leans toward the Sun. The bottom half leans away.

The half that leans toward the Sun has summer. The other half has winter.

Earth's spin axis is tilted, and NASA explains that it always points the same way as Earth travels around the Sun. Around June, the North Pole leans toward the Sun, so the northern half gets more direct sunlight and longer days: summer. The southern half leans away and gets winter.

Around December it is the South Pole's turn. NASA notes that the poles are on opposite ends of Earth, so they have opposite timing for the seasons. It isn't about being nearer the Sun: Earth is actually closest to the Sun in January.

The declination δ swings from +23.44° in June to −23.44° in December. Because noon altitude is 90° − |φ − δ|, latitudes with the same sign as δ get a higher Sun and, from cos h₀ = −tan φ tan δ, longer days. The two hemispheres therefore always see opposite changes.

The Bureau of Meteorology adds that in winter sunlight is spread over a larger area, so the ground receives less heat. NASA Space Place gives perihelion, about 91.4 million miles, in January and aphelion, about 94.5 million, in July, showing distance is not the cause. The model uses a circular orbit and top-of-atmosphere sunlight only.

In Australia, summer comes in December and January. Winter comes in June and July.

In the far south of Australia, winter days are short. Summer days are long.

The Bureau of Meteorology says Australia's summer solstice is around December 21, its longest day, and the winter solstice around June 21, its shortest. In Hobart, in the far south, winter solstice brings only about 9 hours of daylight and summer solstice about 15.

In Australia's tropical north it is different: average temperatures change less between 'winter' and 'summer', so the year is divided into a wet season and a dry season instead.

The Bureau of Meteorology uses calendar-month seasons for Australia, with winter from 1 June, because they fit Australian temperatures better than solstice and equinox dates. Southern Australia has distinct seasons, with Hobart ranging from about 9 to 15 hours of daylight; the tropical north uses a wet season (October–April) and a dry season (May–September).

The model's Sydney figures, 9.7 h in June and 14.3 h in December, sit between Hobart's 9 to 15 hours and Darwin's 11.5 to 12.5 hours.