The Sun’s daily arc across your sky shifts gradually northward from December to June, then back south from June to December.
The Sun’s arc through the daytime sky, at the equinoxes and solstices, for places at latitude 40°. The Sun’s disk is shown at hourly intervals, and is exaggerated 4 times in size. The projection, based simply on altitude and azimuth, makes the horizon appear as a straight line; if instead we used a polar projection based on the middle of the sky, the horizon would be a circle and the Sun’s arcs would be less curved.
For a higher latitude, the Sun’s arcs are lower. For the north pole, all would be straight lines, along the horizon at the equinoxes, above and below it at the solstices. For places on the equator, the equinox arc would curve up to altitude 90°. For south-hemisphere latitudes, the meridian point on the horizon would be “north.”
The cause is that the spinning Earth maintains an attitude tilted by 23.4° to the ecliptic plane in which it travels around the Sun. (This is called the obliquity of Earth’s rotational axis.)
Earth at the equinoxes and solstices. The Sun’s size is exaggerated 20 times, the Earth’s 1500 times.
Earth at the equinoxes and solstices, seen from the direction of the Sun (actually, from 35° north of the Sun’s viewpoint, so as to be able to show some of Earth’s night side) and from a distance of 60 Earth-radii. Because the Earth is at four widely separated points around the ecliptic plane, its attitudes differ as seen from the Sun. The thick arrow is a “rail” along which the planet is riding in its orbit at its speed of about 2,574,000 km per day; each projecting part of the arrow is a distance the Earth advances in one minute (around 1,800 km). An arrow above the equator shows how far Earth rotates in one hour (15°) around its axis (shown by sticks at the poles). A trident represents the vertical beam of sunlight, striking where the Sun is at the zenith at noon. Earth is shown at 12h UT on the day of the event: the hour when the 0° longitude line (Greenwich) faces the Sun.
At the March equinox, the Sun, appearing to travel along the ecliptic, reaches the point (in Pisces) where it crosses the celestial equator into the northern celestial hemisphere. It passes overhead at noon for all places along Earth’s equator. Night and day are everywhere of equal length – hence “equi-nox.” The hemispheres receive equal sunlight. Earth’s two poles are equidistant from the Sun, the north pole leaning backward from the direction of travel. This is the spring or vernal equinox for our northern hemisphere; but it is the fall or autumn equinox for the southern.
At the June (or northern summer) solstice, Earth’s north pole is tilted toward the Sun at the maximal 23.4° angle. The Sun as seen from Earth reaches the point on the ecliptic that is farthest north (23.4°) of the celestial equator. It passes overhead for places along the Tropic of Cancer. For the north hemisphere, days are longest, nights shortest.
At the September equinox (autumn or fall equinox for the northern hemisphere), the Sun reaches the other point (in Virgo) where it crosses the equator, into the southern celestial hemisphere. It again passes overhead along Earth’s equator. The two poles are again equidistant from the Sun, the north pole now leaning forward. Again the hemispheres receive equal sunlight, and days and nights are equal.
At the December (or northern winter) solstice, the Sun appears farthest south of the celestial equator. It passes overhead along the Tropic of Capricorn. Earth’s north pole leans maximally away from the Sun. For northern lands, days are shortest.
For our north pole, the Sun at the equinoxes has no altitude, appearing to run all around the horizon; it is in the sky permanently from March to September; and not at all from September to March. For lands poleward of the Arctic and Antarctic circles, these 24-hour days and 24-hour nights persist for up to 6 months. For lands between the tropics, the Sun is sometimes north and sometimes south of overhead. For north-temperate latitudes (between Arctic circle and tropic), the Sun makes a slanting arch which always passes south of overhead. From March to September this arch is longer and higher, so that the Sun is in the sky more than half the time; it is higher than average at each time of day; its light arrives through less atmosphere and at a steeper angle, so is more concentrated per area of ground.
These four cardinal events fall about 2/3 through March, June, September, and December. Here are dates with Universal Times (rounded to the hour):
2020: Mar 20 04, Jun 20 22, Sep 22 14, Dec 21 10
2021: Mar 20 10, Jun 21 04, Sep 22 19, Dec 21 16
2022: Mar 20 16, Jun 21 09, Sep 23 01, Dec 21 22
2023: Mar 20 21, Jun 21 15, Sep 23 07, Dec 22 03
2024: Mar 20 03, Jun 20 21, Sep 22 13, Dec 21 09
2025: Mar 20 09, Jun 21 03, Sep 22 18, Dec 21 15
2026: Mar 20 15, Jun 21 08, Sep 23 00, Dec 21 21
2027: Mar 20 20, Jun 21 14, Sep 23 06, Dec 22 03
2028: Mar 20 02, Jun 20 20, Sep 22 12, Dec 21 08
2029: Mar 20 08, Jun 21 02, Sep 22 18, Dec 21 14
2030: Mar 20 14, Jun 21 08, Sep 22 23, Dec 21 20:
Some of the variation is due to leap days, but the average dates have shifted over the centureis, For instance, the March equinox will after 2044 sometimes fall on March 19; the September equinox was before 1931 sometimes on Sep. 24.
Daylight begins and ends when the top of the Sun is visible; also the Sun’s apparent height is raised by refraction when it is near the horizon. For these reasons, the actual dates when day and night hours are most nearly equal are slightly before the spring equinox and after the autumn equinox; and the total of day-time in the year is longer than night-time.
There is more in the Astronomical Companion section SEASONS.