To read the numbers
- Measured in arcminutes (′): one degree of sky divided into 60.
- The full Moon is about 31′ across.
- One pixel of StarMap's 4K file is about 2′; one of the HD file, about 3.5′.
- The human eye, at its very best, resolves about 1′.
Results: dates from 1950 to today
| Object | Measurements | Half of cases under | 95% under | Maximum |
|---|---|---|---|---|
| Bright stars The maximum is Alpha Centauri, the fastest-moving bright star in the sky. | 4,504 | 0.06′ | 0.39′ | 2.83′ |
| Moon | 46 | 0.51′ | 0.72′ | 0.83′ |
| Mercury, Venus, Mars, Jupiter and Saturn | 257 | 0.14′ | 1.78′ | 3.20′ |
| Uranus and Neptune | 88 | 0.41′ | 2.56′ | 3.86′ |
Six real dates
| Place | Local date and time | Difference from NASA |
|---|---|---|
| Santiago, Chile | Feb 14, 2020, 10:00 pm | Venus 0.16′ · Uranus 2.81′ |
| Talca, Chile | Dec 24, 2015, 11:30 pm | Moon 0.73′ · Uranus 2.12′ · Neptune 0.07′ |
| Valparaíso, Chile | Jan 1, 2000, midnight | Jupiter 0.13′ · Saturn 0.15′ |
| New York, USA | Jun 16, 2018, 9:30 pm | Moon 0.74′ · Mercury* 6.88′ · Venus 0.03′ · Jupiter 0.12′ · Saturn 0.69′ |
| London, UK | Oct 10, 1995, 8:00 pm | Moon 0.68′ · Jupiter 0.64′ · Saturn 0.33′ · Uranus 0.72′ · Neptune 0.07′ |
| Sydney, Australia | Mar 20, 2024, 9:00 pm | Moon 0.56′ · Jupiter 1.38′ · Uranus 3.05′ |
* Mercury was 0.3° above the horizon, where refraction depends on that night's weather.
How we measured it
- 300 random dates and places between 1900 and 2100, at latitudes from −60° to +65° (Patagonia to Scandinavia).
- For each one, StarMap's real engine (the same files the editor loads) computed the altitude and direction of the Moon, the eight planets and the bright stars.
- The same was computed with Skyfield 1.55 and NASA's JPL DE440s ephemeris. Stars were checked against the Hipparcos catalogue including proper motion.
- Only objects more than 5° above the horizon were compared: lower down, atmospheric refraction depends on each night's weather and no map can know it.
- For dates before 1972, local solar time (UT) was used, which is what clocks showed back then.
Dates between 1900 and 1949
| Object | Measurements | Half of cases under | 95% under | Maximum |
|---|---|---|---|---|
| Bright stars | 2,798 | 0.10′ | 1.33′ | 5.85′ |
| Moon | 35 | 0.22′ | 0.39′ | 0.40′ |
| Mercury to Saturn | 147 | 0.09′ | 1.23′ | 2.65′ |
| Uranus and Neptune | 61 | 0.07′ | 3.66′ | 3.81′ |
Future dates
For dates after 2026, differences reach about 8′, equally for everything. That is not an engine error: nobody knows exactly how the Earth will rotate over the coming decades (its spin speeds up and slows down unpredictably), not even NASA. StarMap uses the same IERS forecast as Skyfield.
What we found and fixed
This comparison helped us first. It found that Uranus and Neptune were off by up to 46′ for dates since 1950 because their tables were missing precession (the slow wobble of Earth's axis), and that the Earth-rotation table ended in 2025. Both were fixed on October 4, 2026, before this page was published, and the numbers above come from the corrected engine.
Before shipping the fix, we generated the Standard, HD, 4K and couple-wallpaper downloads with the old and the new engine and compared them pixel by pixel: only Uranus, Neptune and their labels changed.
Check it yourself
The data, the scripts and the instructions to repeat the comparison are published. If you find a difference that is not listed here, write to contacto@starmap.cl and we will publish it.