Sun Moon Lozenge Applet

Bush Barrow - near Stonehenge, 51.17° N - is a site of the early British Bronze Age (ca. 2000 BC), excavated in 1808 by Sir Richard Colt Hoare and William Cunnington. The barrow contained a male skeleton with rich funerary goods, including a large 'lozenge'-shaped sheet of gold, about 0.5 mm thick.

Source: Wikipedia

The parallelogram (lozenge) is measuring 18 cm across. It might have served as a solar and lunar calendar, using the sight lines pointing in the direction of Sun solstices and lunar standstills.

Azimuth angles of the Sun (red):
    rise and set at summer solstice
    rise and set at winter solstice
    rise and set on May 1 (Beltane), declination about +15°
    rise and set on November 1 (Samain), declination about -15°

Azimuth angles of the Moon (blue):
    northern and southern rise and set at major lunar standstill
    northern and southern rise and set at minor lunar standstill

The azimuth angle az of rise and set depends on the declination δ and the latitude φ. Neglecting atmospheric refraction the angle for a mathematical horizon can be computed by the formula:

cos az = sin δ/ cos φ

At 51.17° N the sizes of the arcs for rise and set are (2000 BC):
    solstices: 80.6°
    solar arc May 1 to November 1: 49.1°
    major lunar standstills: 101.6°
    minor lunar standstills: 61.8°

At 51.17° N the ratio of the lozenge axes NS/EW is 0.821.