How SkyMate Calculates What You Can See

What “visible” means

SkyMate calls an object visible only when all of these hold at the same moment: it is above your horizon, the sky is dark enough for its brightness, it is high enough to see through the haze near the horizon, it is far enough from the Sun, your equipment can show it, nothing local (a building or a tree) blocks it, and the sky is clear. Each condition is calculated separately so SkyMate can explain a “no”.

Positions

Positions of the Sun, Moon and planets are calculated on your device for your place and time. The app uses SkyMate’s own ephemeris code; the website uses the open-source astronomy-engine library (MIT licence). Stars and deep-sky objects use catalogue coordinates (J2000) precessed to the date. Satellites use current orbital elements from CelesTrak with the SGP4 model. Nothing about your location is sent to a server to calculate a position, except city-level coordinates for satellite passes on the website.

Darkness

Darkness comes from the Sun’s altitude. Bright planets and stars show once the Sun is about 6° below the horizon (the end of civil twilight). Faint galaxies and the Milky Way need full astronomical darkness, with the Sun 18° or more below the horizon. The Moon’s phase and altitude add moonlight on top.

On this website

The website uses a deliberately small, documented rule set:

Planets are “easy with your eyes” when they are at least 10° up while the Sun is at least 6° below the horizon, more than 10° from the Sun, and brighter than about magnitude 5.5. Fainter planets are listed as binocular or telescope targets.
Stargazing conditions rate darkness and moonlight only: the hours of full darkness with the Moon down (or under 25% lit). The word is excellent from 5 hours, good from 3, fair from 1. There is no hidden score.
Weather is not used on the website yet, so every answer says “if the sky is clear”. Light pollution, your horizon and your equipment aren’t applied either.
Satellite passes are listed only when the satellite is sunlit and your sky is dark; if current orbits can’t be loaded, no times are shown.

In the SkyMate app

The app adds the parts that need your device: your local forecast (cloud, humidity, transparency), the sky brightness at your place, your saved horizon (Personal Horizon), and your binoculars or telescope. Tonight’s list and the Finder use all of them together.

Limits

Atmospheric conditions vary minute to minute, and forecasts are uncertain. Magnitudes of comets and satellites are estimates. SkyMate shows times to the minute for planning, not as a promise; treat them as “around”.