# Sun, Moon and Earth

37 37 Every time the Moon is closest to the Earth, at her perigees marked P above she is an eighth of the way round the heavens from where she was last time. The Moons furthest positions from the Earth, her apogees labelled A above, likewise divide the heavens into eight. the dance oF the moon Ariadnes eightfold web in the sky The orbital distances of the Earth and Moon undergo periodic changes. This affects the duration and type of eclipses see pages 28 to 34. When the Earth is nearest the Sun, strangely now in chilly January, it is said to be at perihelion when furthest from the Sun it is said to be at aphelion. Similarly, perigee occurs when the Moon is nearest the Earth, whilst apogee finds the Moon furthest from us. The line connecting these two points in the moons slightly elliptical orbit is called the line of apsides. This line or axis, the coming and going of the Moon, itself rotates, completing a cycle every 8.85 years, dividing the zodiac into eight opposite Aapogee, Pperigee. A full moon at perigee appears 30 larger than at apogee. The line of apsides moves anticlockwise around the zodiac by 40o 40 per year, whilst the nodal axis moves clockwise by 19o 20 per year see page 24. Their combined separation is thus 60o per year, a remarkable coincidence causing the nodes and apsides to rendezvous once more after 6 years 360o. Three of these meetings takes 18 years 666 years, coinciding almost exactly with the Saros cycle of 18.03 years. This is why eclipses within consecutive Saros cycles appear of the same type and duration. In nautical almanacs and ephemerides, the position of the Moon is today predicted years in advance using a formula which contains over 1500 separate factors. 36