A clear view of the calculations.

Calculated positions, on your device.

We use Astronomy Engine for celestial positions and events. Latitude, longitude, date and time define the observing situation. The calculations run in your browser; they are not live telescope observations.

The model assumes an unobstructed horizon and an observer at sea level. It cannot account for a hill, building or tree in front of you. Atmospheric refraction is modelled using standard conditions, rather than measured local weather.

Altitude tells you how high. Azimuth tells you where.

Altitude is an angle above the horizon: 0° is the horizon, 90° is directly overhead (the zenith), and a negative number is below the horizon.

Azimuth is the direction around the horizon, measured clockwise from true north. North is 0°, east 90°, south 180° and west 270°. Compass labels are rounded to eight directions. A handheld magnetic compass may need a local magnetic-declination correction to match true north.

Sky View places its markers using these same calculated angles. The dome is an orientation aid: marker sizes are enlarged for readability, and there is no star catalogue or physically scaled planetary disc.

One date means one observing evening.

A selected date refers to that evening and the following morning. We search for events from local noon to the next local noon, normally presenting the night from sunset to the following sunrise. If those events are absent, the noon-to-noon period provides a usable fallback.

All clock times use the selected location’s timezone. Events after midnight are labelled as the next day. Daylight-saving changes can produce shorter or longer sessions; a UTC offset distinguishes repeated local times when clocks move back.

Sunset is the start of a gradual change.

Sunrise and sunset use the apparent upper edge of the Sun with standard atmospheric refraction. Twilight instead follows the geometric centre of the Sun below the horizon.

The same boundaries occur in reverse at dawn. Astronomical darkness is a solar-position definition, not a measurement of sky brightness: the Moon, nearby lighting and atmospheric conditions can still brighten the sky.

At high latitudes, some boundaries may not occur. “No astronomical darkness” and “The Sun does not set” describe different situations. Missing events are shown explicitly rather than assigned invented times.

Observing windows are planning guides.

These thresholds give a consistent way to compare objects. They are product heuristics, not universal definitions of visibility.

Dark-sky overlap uses the end of nautical twilight, so it can begin before astronomical darkness. It does not account for Moon brightness or light pollution. Mercury and Venus remain available in twilight because their useful placement can occur close to sunrise or sunset.

Rise and set events refer to the library’s apparent-horizon model. Sampled above-horizon windows refer to an object’s centre. For the Moon in particular, a disc’s edge can cross the horizon at a different time from its centre.

How to read placement labels.

Higher placement often gives a clearer line of sight, but these labels describe geometry alone. “Well placed” does not mean an object is bright enough to see without binoculars or a telescope.

The Moon: illumination and phase.

Illumination is the proportion of the Moon’s apparent disc lit by the Sun. It is not a cloud forecast, an estimate of landscape brightness or the percentage of the whole Moon receiving sunlight.

The phase name summarizes the Moon’s place in its cycle; illumination comes from the astronomical calculation. The Moon drawing is schematic. It communicates the illuminated fraction and waxing or waning state without claiming the exact tilt of the terminator as seen from your location.

Moonrise and moonset are searched within the observing period. If one does not occur, that does not necessarily mean the Moon remains below the horizon; it may already be above it when the session starts.

Useful precision, not false precision.

Positions are sampled every five minutes. Threshold crossings are interpolated between samples; the highest sampled position is refined with additional calculations nearby. Treat these as approximate planning values. A maximum is the highest position during this session; it can occur at a session boundary, and is not necessarily a meridian transit. Sunrise, sunset and twilight event times use the library’s event searches.

Moving the timeline interpolates object directions between samples without taking a long route across the north/360° boundary. Moon phase and illumination update for the selected time. Apparent magnitude, where shown, uses the astronomical model: lower values mean a brighter object. It does not include your local cloud, haze or horizon obstruction.

Planet cards prioritize practical-altitude windows that overlap darker sky or twilight, followed by other practical windows, then low objects above the horizon and finally objects below it throughout the session. Within each group, higher session maximum altitude comes first, then longer dark-sky overlap.

The initial selected time is the current instant when it falls inside the session. Otherwise it is shortly after astronomical darkness begins, or the middle of the session when no astronomical darkness occurs.

Timezones need a little care.

The bundled PhotoStructure timezone lookup suggests an IANA timezone from coordinates. This compact lookup is approximate and can be wrong near boundaries, coasts or sparsely populated regions. Check the displayed timezone and use the advanced override when needed.

The browser’s timezone database supplies daylight-saving rules. Keeping the browser and operating system current helps keep those rules current too. Timezone selection does not alter the physical position of a planet at a given instant; it changes which local evening you select and how that instant is labelled.

A position is not a promise.

Actual visibility depends on weather, transparency, haze, terrain, buildings, light pollution, Moon brightness, eyesight and observing equipment. Uranus and Neptune in particular should not be interpreted as easy unaided-eye targets. VesperScope does not model these observing conditions.

For the underlying APIs, see the Astronomy Engine JavaScript documentation. For how your location is handled, read our privacy explanation.