2016 D2 | The Moon's size, orbit and the distance to the Sun
Geocentric ephemerides of the Moon for September 2015 are given in the table. Each reading was taken at 00:00 UT. Assume the observer is at the centre of the Earth, and "angular size" means the angular diameter of the object or shadow. A total lunar eclipse occurred this month.
(D2.1) In September 2015, apogee of the lunar orbit is closest to New Moon / First Quarter / Full Moon / Third Quarter. (3)
(D2.2) In September 2015, the ascending node of the lunar orbit on the ecliptic is closest to New Moon / First Quarter / Full Moon / Third Quarter. (4)
(D2.3) Estimate the eccentricity of the lunar orbit from the data. (4)
(D2.4) In the paper, a composite image of the eclipse shows the Moon at several times. A geometric construction (find the centre of the umbra from chords through the shadow edges) gives the umbra's diameter. Take the result: . (8)
(D2.5) The Sun subtends at Earth on the day of the eclipse. Rays from diametrically opposite edges of the solar disc graze the Earth. Calculate the angular size of the penumbra in terms of . (9)
(D2.6) Let be the angular size of the Earth seen from the centre of the Moon. Express (seen from the Earth's centre on the eclipse day) in terms of . (5)
(D2.7) Estimate the radius of the Moon in km. (3)
(D2.8) Estimate the perigee and apogee distances of the Moon. (4)
(D2.9) Use the data for 10 September to estimate the distance to the Sun. (10)
Use km.
| Date | R.A. (h m s) | Dec. (° ′ ″) | Angular size (″) | Phase | Elongation |
|---|---|---|---|---|---|
| Sep 01 | 0 36 46.02 | +3 06 16.8 | 1991.2 | 0.927 | 148.6° W |
| Sep 02 | 1 33 51.34 | +7 32 26.1 | 1974.0 | 0.852 | 134.7° W |
| Sep 03 | 2 30 45.03 | +11 25 31.1 | 1950.7 | 0.759 | 121.1° W |
| Sep 04 | 3 27 28.48 | +14 32 04.3 | 1923.9 | 0.655 | 107.9° W |
| Sep 05 | 4 23 52.28 | +16 43 18.2 | 1896.3 | 0.546 | 95.2° W |
| Sep 06 | 5 19 37.25 | +17 55 04.4 | 1869.8 | 0.438 | 82.8° W |
| Sep 07 | 6 14 19.23 | +18 07 26.6 | 1845.5 | 0.336 | 70.7° W |
| Sep 08 | 7 07 35.58 | +17 23 55.6 | 1824.3 | 0.243 | 59.0° W |
| Sep 09 | 7 59 11.04 | +15 50 33.0 | 1806.5 | 0.163 | 47.5° W |
| Sep 10 | 8 49 00.93 | +13 34 55.6 | 1792.0 | 0.097 | 36.2° W |
| Sep 11 | 9 37 11.42 | +10 45 27.7 | 1780.6 | 0.047 | 25.1° W |
| Sep 12 | 10 23 57.77 | +7 30 47.7 | 1772.2 | 0.015 | 14.1° W |
| Sep 13 | 11 09 41.86 | +3 59 28.8 | 1766.5 | 0.001 | 3.3° W |
| Sep 14 | 11 54 49.80 | +0 19 50.2 | 1763.7 | 0.005 | 7.8° E |
| Sep 15 | 12 39 50.01 | −3 20 03.7 | 1763.8 | 0.026 | 18.6° E |
| Sep 16 | 13 25 11.64 | −6 52 18.8 | 1767.0 | 0.065 | 29.5° E |
| Sep 17 | 14 11 23.13 | −10 09 04.4 | 1773.8 | 0.120 | 40.4° E |
| Sep 18 | 14 58 50.47 | −13 02 24.7 | 1784.6 | 0.189 | 51.4° E |
| Sep 19 | 15 47 54.94 | −15 24 14.6 | 1799.6 | 0.270 | 62.5° E |
| Sep 20 | 16 38 50.31 | −17 06 22.8 | 1819.1 | 0.363 | 73.9° E |
| Sep 21 | 17 31 40.04 | −18 00 52.3 | 1843.0 | 0.463 | 85.6° E |
| Sep 22 | 18 26 15.63 | −18 00 41.7 | 1870.6 | 0.567 | 97.6° E |
| Sep 23 | 19 22 17.51 | −17 00 50.6 | 1900.9 | 0.672 | 110.0° E |
| Sep 24 | 20 19 19.45 | −14 59 38.0 | 1931.9 | 0.772 | 122.8° E |
| Sep 25 | 21 16 55.43 | −11 59 59.6 | 1961.1 | 0.861 | 136.2° E |
| Sep 26 | 22 14 46.33 | −8 10 18.3 | 1985.5 | 0.933 | 150.0° E |
| Sep 27 | 23 12 43.63 | −3 44 28.7 | 2002.0 | 0.981 | 164.0° E |
| Sep 28 | 0 10 48.32 | +0 58 58.2 | 2008.3 | 1.000 | 178.3° E |
| Sep 29 | 1 09 05.89 | +5 38 54.3 | 2003.6 | 0.988 | 167.4° W |
| Sep 30 | 2 07 39.02 | +9 54 16.1 | 1988.4 | 0.947 | 153.2° W |
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Reading resources
Material that covers the physics behind this problem.
- An Introduction to Modern AstrophysicsCarroll & Ostlie
The standard reference for olympiad-level astrophysics. Chapters 1-3 cover the celestial sphere, orbits and the magnitude system.
Textbook - Problems from past IOAAInternational Olympiad on Astronomy and Astrophysics
The official archive of every IOAA paper, including all data analysis rounds since 2007 with their solutions and marking schemes.
Past papers - Measurements and their UncertaintiesHughes & Hase
A short, practical guide to error analysis, least squares fitting and chi-square. Exactly the toolkit of the data round.
Textbook
