Table of Contents¶

Welcome to the documentation for Skyfield, the pure Python astronomy library that is accelerated using NumPy vector math!

Installing Skyfield
- Checking your Skyfield version
- Citing Skyfield
- Changelog
  - Released versions
Examples
- What time is solar noon, when the Sun transits the meridian?
- When will it get dark tonight?
- What phase is the Moon tonight?
- What is the angular diameter of a planet, given its radius?
- When is Venus at its greatest east and west elongations from the Sun?
- Are planets separated by 0° at conjunction and 180° at opposition?
- At what angle in the sky is the crescent Moon?
- When is a body or fixed coordinate above the horizon?
- At what rate is a target moving across the sky?
  - Computing the range rate-of-change
  - Computing angular speed
- What is the right ascension and declination of a point in the sky?
- What latitude and longitude is beneath this right ascension and declination?
- Which geographic location is farther from Earth’s center?
Example Plots
- Charting an apparition of Venus
- Drawing a finder chart for comet NEOWISE
- Plotting satellite altitude during re-entry
Downloading and Using Data Files
- Specifying the download directory
- Turning off the progress bar
Almanac Computation
- Rounding time to the nearest minute
- Risings and settings
- The Seasons
- Phases of the Moon
- Lunar Nodes
- Opposition and Conjunction
- Meridian Transits
- Legacy transit routine
- Legacy rising and setting routines
- Twilight
- Solar terms
- Lunar eclipses
Positions
- Quick reference
- The ICRS reference system and J2000
- Barycentric → Astrometric → Apparent
- Position attributes
- Astrometric right ascension and declination
- Apparent right ascension and declination
- Azimuth and altitude from a geographic position
  - Adjusting altitude for atmospheric refraction
- Comparing positions
- Coordinates in other reference frames
Coordinates
- Cartesian coordinates versus Spherical coordinates
- ICRS and equatorial right ascension and declination
- Altitude and azimuth (‘horizonal’ coordinates)
- Hour Angle and Declination
- ECI versus ECEF coordinates
- Geographic ITRS latitude and longitude
- Ecliptic coordinates
- Galactic coordinates
- Velocity
- Turning coordinates into a position
- Rotation Matrices
Dates and Time
- Ancient and modern dates
- Building and printing UTC
- UTC and your timezone
- UTC and leap seconds
- Date arithmetic
- Date arrays
- Uniform time scales: TAI, TT, and TDB
- UT1 and downloading IERS data
- Setting a Custom Value For ∆T
- Values cached on the Time object
Planets and their moons: JPL ephemeris files
- Choosing an Ephemeris
- Listing the targets that an ephemeris supports
- Making an excerpt of an ephemeris
- Closing the ephemeris file automatically
- Type 1 and Type 21 ephemeris formats
- Ephemeris download links
- Ephemeris bibliography
Stars and Distant Objects
- The Hipparcos Catalog
- Looking up specific stars
- Stars with “nan” positions
- Filtering the star catalog
- Building a single star from its coordinates
- Distances to stars
- Proper motion and parallax
- Position at an epoch besides J2000
Planetary Reference Frames
- Observing a Moon location
- Observing from a Moon location
- Computing the sub-solar point on the Moon
- Computing lunar libration
- Computing a raw rotation matrix
Kepler Orbits
- Comets
- Minor Planets
Earth Satellites
- The TLE format and its rivals
- Checking an element set’s epoch
- Historical satellite element sets
- Finding when a satellite rises and sets
- Generating a satellite position
- Satellite longitude, latitude, and height
- Satellite altitude, azimuth, and distance
- Satellite right ascension and declination
- Find a satellite’s range rate
- Find when a satellite is in sunlight
- Find whether the Earth blocks a satellite’s view
- Avoid calling the observe method
- Detecting Propagation Errors
- Build a satellite with a specific gravity model
- Build a satellite from orbital elements
Searching for the dates of astronomical events
- Finding discrete events
- Finding extrema
  - Finding minima
Skyfield’s Accuracy and Efficiency
- Precession and Nutation
- Polar Motion
- The leap second table
- Bad performance and using 100% CPU
Osculating Orbital Elements
- Generating Elements
- Attributes of OsculatingElements objects
- Reference Planes
Using Skyfield with AstroPy
API Reference
- Version
- Opening files
- Time scales
- Time objects
- Time utilities
- Vector functions
- Planetary ephemerides
- Planetary magnitudes
- Planetary reference frames
- Almanac
- Geographic locations
- Kepler orbits
- Kepler orbit data
- Earth satellites
- Stars and other distant objects
- Astronomical positions
- Reference frames
- Constellations
- Searching
- Osculating orbital elements
- Units
- Trigonometry
API Reference — Opening Files
- The loader class
- Standalone functions
API Reference — Time
- Calendar date
- Timescale, for building and converting times
- The Time object
- Time utilities
API Reference — Vector Functions
API Reference — Planetary Ephemerides
- JPL .bsp ephemeris files
API Reference — Geographic Locations
API Reference — Almanac
API Reference — Earth Satellites
API Reference — Stars and other distant objects
API Reference — Astronomical Positions
- Generic ICRF position
- Position measured from the Solar System barycenter
- Astrometric position relative to an observer
- Apparent position relative to an observer
- Geocentric position relative to the Earth
- Building a position from right ascension and declination
API Reference — Reference Frames
API Reference — Planetary reference frames
API Reference — Kepler Orbits
API Reference — Orbital Elements
API Reference — Units
- Formatting angles
API Reference — Trigonometry
Design Notes
- Position classes with coordinate methods
- Angle Classes
- Importing NumPy
- Rotation matrices or state transformation matrices?
- Cross products
Bibliography