Module 06
The fraction of the Moon that was lit on the night you were born, how far away it was, and — the part almost every version of this gets wrong — which way round it appeared from the hemisphere you were in.
Amsterdam, Netherlands
Between the extremes of the orbit, which range from about 356,500 to 406,700 kilometres.
Lit on the left, as it hung from the northern hemisphere. From the southern hemisphere the same phase is mirrored — lit on the right.
Nineteen years is 235 lunations to within a couple of hours, so the Moon returns to nearly the same phase on the same date. Nearly: nineteen calendar years is a whole number of days and 235 lunations is not, and the two drift apart.
| Date | Lit | Drift |
|---|---|---|
| 15 JUN 2009 | 55.3% Third quarter | 3.4 pts |
| 15 JUN 2028 | 47.9% Third quarter | 10.7 pts |
| 15 JUN 2047 | 49.2% Third quarter | 9.4 pts |
Measured across two hundred birth dates: the illuminated fraction usually comes back within a couple of percentage points, and the phase keeps the same name about nine times out of ten.
Phase is very nearly a global fact — two people on opposite sides of the Earth see the same fraction lit at the same moment. What differs is the orientation, which is why this asks where you were.
The dark plains are the maria, at their catalogued positions and roughly their true sizes, flattening towards the rim exactly as they do through a telescope. Their outlines here are ellipses; a mare is a ragged flood of basalt, and this is a plate rather than a survey. Five craters are marked, at true size — which is smaller than most illustrations admit.
Below the equator the face is turned through half a circle rather than mirrored. For the phase alone the two are indistinguishable, because the lit shape is symmetric about the horizontal. For the maria they are not, and a mirrored Moon is one that has never risen anywhere.
The drawing shows the disc as it appears when the Moon is high in the sky. Closer to the horizon the whole thing tilts, by an amount that depends on the time of night as well as the latitude.
Age is measured by finding the new moon that actually preceded the moment. The month between two new moons runs from about 29.27 to 29.83 days, so dividing by an average would be wrong by up to several hours.
Positions from the astronomy-engine implementation of the VSOP87 and ELP2000 series.