Sunset tomorrow

Sunset is the moment the Sun's top edge disappears below a flat horizon. Photographers care more about what comes before and after:

• Golden hour — while the Sun is less than about 6° high. Light travels through more air, blue is scattered away, and shadows grow long and warm. Near the equator it lasts about 25 minutes; in London in June, well over an hour.
• Blue hour — just after sunset, with the Sun 4–6° below the horizon: indirect, deep-blue light that makes city lights glow.
• Civil, nautical and astronomical dusk — the Sun 6°, 12° and 18° below the horizon. After astronomical dusk the sky is fully dark (if there is no moon or city glow).

Sunsets look redder than noon sunlight because sunlight crosses up to about 40 times more air near the horizon, and air scatters blue light far more than red. Dust, smoke and haze — after a dust storm or crop burning, for instance — deepen the colours. On a very clear sea horizon you may catch a green flash: a second or two of green at the Sun's last edge, caused by the atmosphere bending green light slightly more than red.

If sunset simply tracked day length, the earliest sunset would fall on the December solstice. Instead, in the northern hemisphere the earliest sunset comes in early to mid-December, and evenings are already getting longer by the shortest day. The reason is that solar noon itself drifts through the year.

Earth's orbit is an ellipse and its axis is tilted, so the Sun's position doesn't advance evenly across the sky. The difference between clock noon and the moment the Sun is highest — the equation of time — swings between about 16 minutes early (early November) and 14 minutes late (mid-February). Around the solstices, day length is barely changing, so this drift in solar noon wins and pulls sunset away from the solstice date. The closer you are to the equator, the smaller the change in day length and the bigger the effect: in Chennai the earliest sunset is weeks away from the solstice.