Phase angle of a celestial body in degrees at a given datetime.

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Overview

Returns the phase angle of a celestial body at a given location and datetime. The phase angle is the angle at the body between the directions to the Sun and to the observer. A phase angle of 0° means the body is fully illuminated as seen from the observer (full phase); 180° means the body is between the observer and the Sun (new phase, not visible). For the Moon this mirrors the illumination percentage — for planets it indicates how much of the sunlit hemisphere faces Earth.

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Request parameters

Name	Type	In	Required	Description	Example
body	Choice	Body	Yes	The celestial body to query. Must be one of the Skyfield keys returned by the /astra/bodies/ endpoint (e.g. SUN, MOON, MARS). Case-sensitive.
date1	Date	Body	Yes	The start date (or single query date) for the computation. Accepted formats: YYYY-MM-DD for date-only fields, ISO 8601 datetime string (YYYY-MM-DDTHH:MM) for datetime fields.
lat	Float	Body	Yes	Observer latitude in decimal degrees. Valid range: -90.0 (South Pole) to 90.0 (North Pole). Used to compute the observer's position on Earth's surface for all sky calculations.
lon	Float	Body	Yes	Observer longitude in decimal degrees. Valid range: -180.0 to 180.0. Negative values are west of the Prime Meridian, positive values are east.
timezone_slug	Choice	Body	Optional	Slug of a timezone record used to localise event datetimes in the response (e.g. america-new-york). When omitted, the timezone is derived automatically from the supplied coordinates. Obtain valid timezone slugs from the timezone list endpoint.

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Response fields

Name	Type	Nullable	Description	Example
angle	Float	—	Phase angle of the body in degrees. 0° means fully illuminated (opposition/full); 180° means new or unlit.	45.3
body	String	—	Skyfield key of the requested celestial body.	MOON
date1	Datetime	—	Observation datetime echoed from the request.	2025-06-21T14:00:00
lat	Float	—	Observer latitude echoed from the request.	48.428333
lon	Float	—	Observer longitude echoed from the request.	-123.364722
tz	String	—	Resolved IANA timezone name.	America/Vancouver