The Sun’s path

How to use it

Pick a town, then a day between 1901 and 2099, or press one of the four buttons to jump to that year’s equinox or solstice. The drawing is the sky seen from above, like a map: the outer circle is the horizon, the centre is straight overhead, north is at the top and east to the right. The gold arc is the Sun’s path on the chosen day, with a dot and a number on each whole hour by the clock, and the fainter arcs are the June solstice, the March equinox and the December solstice of the same year. The bearings of sunrise and sunset are written on the rim. The slider moves the Sun through the day and says how high it stands, which way it lies and, once it has set, whether the sky is in civil, nautical or astronomical twilight or fully dark.

Reading the numbers, with an example

Height is in degrees above the horizon. Bearing is measured round from north through east, so 90° is due east, 180° due south and 270° due west. For anywhere north of the tropics the noon height follows a simple rule: at an equinox it is 90° minus the latitude, at the June solstice about twenty-three and a half degrees more, and at the December solstice the same amount less. South of the tropics the two solstices swap round.

London lies at about 51.5° north, so its noon Sun stands about 38° up at the equinoxes, about 62° in late June and only about 15° in late December. A garden bed that gets sun over a fence in summer can lose it all winter, and the December arc on the drawing shows how low the Sun passes. The list under the drawing gives the chosen day in words: sunrise and sunset with their times and bearings, the highest and lowest points, the length of the day, and how far the sunrise swings along the horizon between the two solstices.

Why the path moves through the year

The Earth’s axis is tilted by about twenty-three and a half degrees, so over a year the Sun’s daily circle slides north and south of the celestial equator. At the equinoxes the Sun rises almost exactly due east and sets due west everywhere on Earth, and at noon it stands as far below the point overhead as the place is from the equator. North of the tropics, around the June solstice it rises well north of east, climbs highest and sets well north of west; in December it rises south of east and stays low. The further from the equator, the wider the swing: in London the sunrise moves through about eighty degrees of horizon in a year, at the equator through about forty-seven. Beyond the polar circles there are days when the Sun does not set at all and days when it does not rise.

South of the equator the picture turns round: the noon Sun lies to the north, and the June solstice gives the low, short arc of midwinter. Between the tropics the Sun stands straight overhead at noon on two days a year, and in the weeks between those two days it passes on the other side of overhead, to the north for a place north of the equator.

Clock noon is not the Sun’s noon

The Sun is highest when it crosses the meridian, the line from due north to due south through the point overhead, and that is rarely 12:00 by the clock. A time zone is wide, so the Sun’s noon comes later in its west than in its east; summer time moves it on another hour; and because the Earth’s axis is tilted and its orbit is an ellipse, the Sun’s noon drifts through the year by up to about a quarter of an hour either way. That drift, the equation of time, is also why in the northern hemisphere the earliest sunset of the year comes some days before the December solstice and the latest sunrise some days after it.

How it is worked out

For every five minutes of the chosen day the page takes the Sun’s position from the same astronomy as the rest of the site, as seen from the place and allowing for the air bending its light near the horizon, and turns it into a height above the horizon and a bearing from north. Sunrise and sunset are the minutes the top of the Sun meets the horizon, and noon is when the Sun crosses the meridian. The day runs from the Sun’s own midnight to the next, and times are given by the clock of the place, or by this device’s clock when the place’s time zone is not known. Hills, buildings and trees are not included, so a sunrise behind a ridge comes later and further round than this.

What it claims

The drawing describes where the light comes from at a given minute, which is the same geometry that builders, gardeners and photographers use, and that old traditions used to face a door or a temple to the rising Sun on a particular day. It says where the Sun is and predicts nothing.

Sunrise, sunset and the stages of twilight for every day of the year are on Daylight. The exact minutes of the equinoxes and solstices are on The year, the daylight cut into twelve planetary hours on Hours, and the planets that come out once the Sun has gone on Tonight.