The sky on September 20, 2011 — as it really was

The day’s dossier, from real data

Checkable facts with a tag and a source; the classical reading is in the chart.

1. The Moon that day [EXACT]

   last quarter, 51% lit; the Moon was 392,510 km away.

   astronomy-engine
2. How far Earth has travelled since [EXACT]

   15 orbits of the Sun — 14,135 million km, that is 18,386 return trips to the Moon; 5,494 rotations of Earth; with the Sun — 109.2 billion km around the Galactic centre, 4.2 light-days.

   29.78 km/s along the orbit; 230 km/s around the Galactic centre
3. Mars then and now [EXACT]

   On that day Mars was 281 million km away; today it is 245 million km (closer by 1.1×).

   astronomy-engine; checked against JPL Horizons for 2026-09-14
4. Solar cycle [ESTIMATE]

   That day fell in solar cycle 24, on the rise to maximum (25% through the cycle).

   SILSO smoothed minima
5. When the sky repeats [EXACT]

   Mars returns to that day’s point of its orbit on October 7, 2026; Jupiter — in 2035; a full moon exactly on this date — September 20, 2051

   sidereal periods: Mars 686.98 d, Jupiter 11.862 yr
6. How far the sky has drifted since [EXACT]

   The vernal point has shifted 756″ since that day; the real constellations now sit 24° away from the astrologers’ “signs”.

   precession 50.29″/yr
7. Picture of the day [EXACT]

   NASA Astronomy Picture of the Day for this date: apod.nasa.gov/apod/ap110920.html

   NASA APOD