Methodology
Astrotempo calculates everything about the sky itself, in your browser or at build time, from published algorithms and public data. Nothing is estimated by hand. This page explains how, and how the results are checked.
The astronomy engine
Positions of the Sun, Moon and planets, rise, set and transit times, twilight, Moon phases, equinoxes, solstices, eclipses, oppositions and elongations are calculated with Astronomy Engine by Don Cross (MIT licence). Its author verified it against the US Naval Observatory’s NOVAS software and NASA JPL Horizons. Astrotempo repeats that check on every build (see below).
Conventions:
- Rise and set refer to the upper edge of the Sun or Moon touching a level horizon with average atmospheric refraction, as in almanacs. The site uses the place’s elevation from GeoNames when it is known. Real horizons with mountains or buildings can shift the times by several minutes.
- Twilight follows the US Naval Observatory definitions: civil, nautical and astronomical twilight end when the Sun’s centre is 6°, 12° and 18° below the horizon. See the twilight guide.
- Times are shown in the place’s time zone (IANA time zone database), including daylight saving time.
- “Tonight” means the night that starts on the evening of the selected date. Before local noon, the site shows the night that is still going on.
Planet visibility
A planet is listed as visible tonight when it is at least 10° above the horizon while the Sun is at least 6° below it (after civil dusk and before civil dawn). Planets brighter than magnitude 5 are marked as visible to the eye, and fainter ones (in practice Uranus and Neptune) as needing binoculars or a telescope. The best time is when the planet is highest within that window. These thresholds are a choice: a planet very low in bright twilight can sometimes be seen, and a high one can be hidden by local obstacles.
Stars, constellations and deep-sky objects
- Stars: the HYG database v4 (CC BY-SA 4.0), which combines the Hipparcos, Yale Bright Star and Gliese catalogues. The 3D sky view draws stars down to magnitude 6.5, with colours from their B−V colour index.
- Constellation lines: Stellarium’s western sky culture.
- Deep-sky objects: OpenNGC (CC BY-SA 4.0).
The 3D sky also models how the atmosphere dims and reddens stars near the horizon (extinction) and the colour of the twilight sky. These effects are for realism. Numbers in tables do not depend on them.
Meteor showers
Shower data come from the IAU Meteor Data Center’s list of established showers (Jenniskens et al. 2020). For each shower the site takes the median of the established solutions. Peak dates are calculated from the solar longitude of the peak, so they move with the calendar from year to year. Peaks are forecasts: activity can come earlier or later and rates vary from year to year.
Eclipses
Eclipse times are calculated with Astronomy Engine. Eclipse types (total, annular, hybrid, partial, penumbral) are taken from NASA’s eclipse tables by Fred Espenak and Jean Meeus, because Astronomy Engine does not distinguish hybrid solar eclipses.
For the total solar eclipse of 2 August 2027 the site also calculates the local circumstances for any place (contact times, duration of totality, fraction of the Sun covered) and the path of totality on the WGS 84 ellipsoid, at sea level. Astronomy Engine uses the Moon’s mean radius, so durations come out a few seconds longer than NASA’s. Mountains and valleys on the lunar limb are not modelled, and weather is not included.
Comets and asteroids
Orbital elements of bright comets and asteroids come from the NASA/JPL Small-Body Database once a day. Positions are propagated from these elements as a two-body orbit, which is accurate for weeks to months around the epoch of the elements but ignores planetary perturbations beyond that. Comet brightness is notoriously hard to predict. Magnitudes are estimates from JPL’s photometric parameters.
Live data
- Cloud cover: hourly forecasts from MET Norway’s Locationforecast API, requested by your browser for your coordinates (rounded to 2 decimals) when you open the tonight page.
- Aurora: NOAA Space Weather Prediction Center’s Kp forecast and the OVATION aurora forecast, also requested by your browser.
- Small bodies: updated daily from NASA/JPL as described above.
If a live source is unavailable, the page says so and shows everything else.
How results are checked
Every build runs an automatic test suite that compares the site’s calculations with independent reference data. A build that fails a test is not published.
| Check | Reference | Limit | Worst |
|---|---|---|---|
| Positions of the Sun, Moon and 8 planets (RA/Dec, altitude/azimuth), 20 cities × 24 times: 4,800 positions | NASA JPL Horizons | 1′ | 0.3′ |
| Sun and Moon rise, transit and set, 20 cities, October 2026 | NASA JPL Horizons | 2 min | 1.0 min |
| Principal Moon phases 2026–2035 (495) | USNO | 1.5 min | 1.3 min |
| Equinoxes and solstices 2026–2035 (40) | USNO | 1.5 min | 1.3 min |
| Solar and lunar eclipses 2026–2035 (22 + 23): same events and types | NASA eclipse tables | 1 min | passed |
| Path of the total solar eclipse of 2 Aug 2027: central line and limits | NASA eclipse web site (Fred Espenak) | 3 km / 5 km | passed |
| Local solar eclipse contact times, eclipses of 2017–2026 (sea level) | USNO | 10 s | passed |
| Comets and asteroids, up to 90 days from the epoch | NASA JPL Horizons | 5′ | passed |
Part of each difference comes from rounding in the reference itself: Horizons rise and set times were requested at one-minute steps, and the USNO publishes times to the minute.
Licences and credits
Every dataset, image and library is listed with its licence and credit on the data sources page.
Errors
If something looks wrong, please write to hello@astrotempo.com with the place, the date and what you expected. See contact.
Last updated October 10, 2026