The Sun and the Moon around the eclipse of 2008
- Round
- Observation
- Points
- 4
- Level
- Silver
- Taught in
- Section O1.9
Adapted from IOAA 2025 (Mumbai), sky map problem 1. We have restated two parts of it in our own words and kept the original data. The original asks for the Sun and the Moon to be drawn on a printed sky map, and we ask for their positions in words. The list of stars in the first part comes from another part of the original, which asks for them to be marked on the same map. The solution was last checked on 18 October 2026.
A total eclipse of the Sun took place on 1 August 2008, and at one place on Earth totality came at local noon.
- Give the position of the Sun among the stars at that moment. Name its constellation, and say which of these stars it was closest to: β Aur, δ Cep, δ Cnc, δ Cet, δ Gem, β CVn, α Lyn and β Per. (1 point)
- Describe the position of the Moon among the stars at local noon at the same place on 28 July 2008, and its shape. Which side of it was lit? (3 points)
- During a total eclipse the Moon is in front of the Sun, so both are at the same place on the sky, on or very close to the ecliptic. The Sun passes Regulus, at ecliptic longitude 150°, around 23 August (Section O1.7), and it moves about 1° a day along the ecliptic, so on 1 August, 22 days earlier, it was near longitude
- By the table of Section O1.7 the Sun is in Cancer from 21 July to 11 August, so it was in Cancer. Of the eight stars, δ Cnc, at ecliptic longitude 128.7°, lies within 0.1° of the ecliptic, and the Sun was only 0.7° from it.
- On 28 July, four days before the eclipse, the Moon was west of the Sun. It moves eastward among the stars by about 13.2° a day and the Sun by about 1.0°, so it gains on the Sun by about 12.2° a day, and four days earlier it was about 49° west of it. The full calculation gives 55°, because in those days the Moon was near perigee and moving faster than average. That puts it at ecliptic longitude about 71°, in Taurus, 4.8° north of the ecliptic and about 10° north of Aldebaran.
- A Moon 55° from the Sun is a crescent, with an illuminated fraction of
AnswerOn 1 August 2008 the Sun was in Cancer, 0.7° from δ Cnc. On 28 July the Moon was in Taurus, about 10° north of Aldebaran, as a waning crescent about one fifth lit, with its bright side towards the east.
This problem also appears among the olympiad problems of Section O1.9, Problems for Chapter O1, which teaches the ideas it uses. Equation and figure numbers refer to the course.