Skyfield: HomeTable of ContentsChangelogAPI Reference

Almanac Computation

The highest-level routines in Skyfield let you search back and forward through time for the exact moments when the Earth, Sun, and Moon are in special configurations.

They all require you to start by loading up a timescale object and also an ephemeris file that provides positions from the planets:

from skyfield import api

ts = api.load.timescale()
eph = api.load('de421.bsp')

Then, load the “almanac” module.

from skyfield import almanac

Note that almanac computation can be slow and expensive. To determine the moment of sunrise, for example, Skyfield has to search back and forth through time asking for the altitude of the Sun over and over until it finally works out the moment at which it crests the horizon.

Rounding time to the nearest minute

If you compare almanac results to official sources like the United States Naval Observatory, the printed time will often differ because the Naval Observatory results are rounded to the nearest minute — any time with :30 or more seconds at the end gets named as the next minute.

If you try to display a date that needs to be rounded to the nearest minute by simply stopping at %M and leaving off the %S seconds, the output will be one minute too early. For example, the Naval Observatory would round 14:59 up to :15 in the following date.

t = ts.utc(2018, 9, 10, 5, 14, 59)
dt = t.utc_datetime()
print(dt.strftime('%Y-%m-%d %H:%M'))
2018-09-10 05:14

To do the same rounding yourself, simply add 30 seconds to the time before truncating the seconds.

from datetime import timedelta

def nearest_minute(dt):
    return (dt + timedelta(seconds=30)).replace(second=0, microsecond=0)

dt = nearest_minute(t.utc_datetime())
print(dt.strftime('%Y-%m-%d %H:%M'))
2018-09-10 05:15

The results should then agree with the tables produced by the USNO.

The Seasons

Create a start time and an end time to ask for all of the equinoxes and solstices that fall in between.

t0 = ts.utc(2018, 1, 1)
t1 = ts.utc(2018, 12, 31)
t, y = almanac.find_discrete(t0, t1, almanac.seasons(eph))

for yi, ti in zip(y, t):
    print(yi, almanac.SEASON_EVENTS[yi], ti.utc_iso(' '))
0 Vernal Equinox 2018-03-20 16:15:27Z
1 Summer Solstice 2018-06-21 10:07:18Z
2 Autumnal Equinox 2018-09-23 01:54:06Z
3 Winter Solstice 2018-12-21 22:22:44Z

The result t will be an array of times, and y will be 0 through 3 for the Vernal Equinox through the Winter Solstice.

If you or some of your users live in the Southern Hemisphere, you can use the SEASON_EVENTS_NEUTRAL array. Instead of naming specific seasons, it names the equinoxes and solstices by the month in which they occur — so the March Equinox, for example, is followed by the June Solstice.

Phases of the Moon

The phases of the Moon are the same for everyone on Earth, so you don’t need to specify the longitude and latitude of your location. Simply ask for the current phase of the Moon as an angle, where 0° is New Moon and 180° is Full:

t = ts.utc(2020, 11, 19)
phase = almanac.moon_phase(eph, t)
print('Moon phase: {:.1f} degrees'.format(phase.degrees))
Moon phase: 51.3 degrees

Or you can have Skyfield search over a range of dates for the moments when the Moon reaches First Quarter, Full, Last Quarter, and New:

t0 = ts.utc(2018, 9, 1)
t1 = ts.utc(2018, 9, 10)
t, y = almanac.find_discrete(t0, t1, almanac.moon_phases(eph))

print(t.utc_iso())
print(y)
print([almanac.MOON_PHASES[yi] for yi in y])
['2018-09-03T02:37:24Z', '2018-09-09T18:01:28Z']
[3 0]
['Last Quarter', 'New Moon']

The result t will be an array of times, and y will be a corresponding array of Moon phases with 0 for New Moon and 3 for Last Quarter. You can use the array MOON_PHASES to retrieve names for each phase.

Lunar Nodes

The Moon’s ascending node and descending node are the moments each lunar month when the Moon crosses the plane of Earth’s orbit and eclipses are possible.

t0 = ts.utc(2020, 4, 22)
t1 = ts.utc(2020, 5, 22)
t, y = almanac.find_discrete(t0, t1, almanac.moon_nodes(eph))

print(t.utc_iso())
print(y)
print([almanac.MOON_NODES[yi] for yi in y])
['2020-04-27T17:54:17Z', '2020-05-10T09:01:42Z']
[1 0]
['ascending', 'descending']

Opposition and Conjunction

The moment at which a planet is in opposition with the Sun or in conjunction with the Sun is when their ecliptic longitudes are at 0° or 180° difference.

t0 = ts.utc(2019, 1, 1)
t1 = ts.utc(2021, 1, 1)
f = almanac.oppositions_conjunctions(eph, eph['mars'])
t, y = almanac.find_discrete(t0, t1, f)

print(t.utc_iso())
print(y)
['2019-09-02T10:42:14Z', '2020-10-13T23:25:47Z']
[0 1]

The result t will be an array of times, and y will be an array of integers indicating which half of the sky the body has just entered: 0 means the half of the sky west of the Sun along the ecliptic, and 1 means the half of the sky east of the Sun. This means different things for different bodies:

Meridian Transits

Every day the Earth’s rotation swings the sky through nearly 360°, leaving the celestial poles stationary while bringing each star and planet in turn across your meridian — the “line of longitude” in the sky above you that runs from the South Pole to the North Pole through the zenith point directly above your location on Earth. You can ask Skyfield for the times at which a body crosses your meridian, and then the antimeridian on the opposite side of the celestial globe:

bluffton = api.wgs84.latlon(+40.8939, -83.8917)

t0 = ts.utc(2020, 11, 6)
t1 = ts.utc(2020, 11, 7)
f = almanac.meridian_transits(eph, eph['Mars'], bluffton)
t, y = almanac.find_discrete(t0, t1, f)

print(t.utc_strftime('%Y-%m-%d %H:%M'))
print(y)
print([almanac.MERIDIAN_TRANSITS[yi] for yi in y])
['2020-11-06 03:32', '2020-11-06 15:30']
[1 0]
['Meridian transit', 'Antimeridian transit']

Some astronomers call these moments “upper culmination” and “lower culmination” instead.

Observers often think of transit as the moment when an object is highest in the sky, which is roughly true. But at very high precision, if the body has any north or south velocity then its moment of highest altitude will be slightly earlier or later.

Bodies near the poles are exceptions to the general rule that a body is visible at transit but below the horizon at antitransit. For a body that’s circumpolar from your location, transit and antitransit are both moments of visibility, when it stands above and below the pole; and objects close to the opposite pole will always be below the horizon, even as they invisibly transit your line of longitude down below your horizon.

Sunrise and Sunset

Because sunrise and sunset differ depending on your location on the Earth’s surface, you will need to specify a latitude and longitude.

Then you can create a start time and an end time and ask for all of the sunrises and sunsets in between. Skyfield uses the official definition of sunrise and sunset from the United States Naval Observatory, which defines them as the moment when the center — not the limb — of the sun is 0.8333 degrees below the horizon, to account for both the average radius of the Sun itself and for the average refraction of the atmosphere at the horizon.

t0 = ts.utc(2018, 9, 12, 4)
t1 = ts.utc(2018, 9, 13, 4)
t, y = almanac.find_discrete(t0, t1, almanac.sunrise_sunset(eph, bluffton))

print(t.utc_iso())
print(y)
['2018-09-12T11:13:13Z', '2018-09-12T23:49:38Z']
[1 0]

The result t will be an array of times, and y will be 1 if the sun rises at the corresponding time and 0 if it sets.

If you need to provide your own custom value for refraction, adjust the estimate of the Sun’s radius, or account for a vantage point above the Earth’s surface, see Risings and Settings to learn about the more versatile risings_and_settings() routine.

Note that a location near one of the poles during polar summer or polar winter will not experience sunrise and sunset. To learn whether the sun is up or down, call the sunrise-sunset function at the time that interests you, and the return value will indicate whether the sun is up.

far_north = api.wgs84.latlon(89, -80)
f = almanac.sunrise_sunset(eph, far_north)
t, y = almanac.find_discrete(t0, t1, f)

print(t.utc_iso())  # Empty list: no sunrise or sunset
print(f(t0))        # But we can ask if the sun is up

print('polar day' if f(t0) else 'polar night')
[]
True
polar day

Twilight

An expanded version of the sunrise-sunset routine named dark_twilight_day() returns a separate code for each of the phases of twilight:

  1. Dark of night.
  2. Astronomical twilight.
  3. Nautical twilight.
  4. Civil twilight.
  5. Daytime.

You can find a full example of its use at the When will it get dark tonight?.

Risings and Settings

Skyfield can compute when a given body rises and sets. The routine is designed for bodies at the Moon’s distance or farther, that tend to rise and set about once a day. But it might be caught off guard if you pass it an Earth satellite that rises several times a day; for that case, see Historical satellite element sets.

Rising and setting predictions can be generated using the risings_and_settings() routine:

t0 = ts.utc(2020, 2, 1)
t1 = ts.utc(2020, 2, 2)
f = almanac.risings_and_settings(eph, eph['Mars'], bluffton)
t, y = almanac.find_discrete(t0, t1, f)

for ti, yi in zip(t, y):
    print(ti.utc_iso(), 'Rise' if yi else 'Set')
2020-02-01T09:29:16Z Rise
2020-02-01T18:42:57Z Set

As with sunrise and sunset above, 1 means the moment of rising and 0 means the moment of setting.

The routine also offers some optional parameters, whose several uses are covered in the following sections.

Computing your own refraction angle

Instead of accepting the standard estimate of 34 arcminutes for the angle by which refraction will raise the image of a body at the horizon, you can compute atmospheric refraction yourself and supply the resulting angle to horizon_degrees. Note that the value passed should be a small negative angle. In this example it makes a 3 second difference in both the rising and setting time:

from skyfield.earthlib import refraction

r = refraction(0.0, temperature_C=15.0, pressure_mbar=1030.0)
print('Arcminutes refraction for body seen at horizon: %.2f\n' % (r * 60.0))

f = almanac.risings_and_settings(eph, eph['Mars'], bluffton, horizon_degrees=-r)
t, y = almanac.find_discrete(t0, t1, f)

for ti, yi in zip(t, y):
    print(ti.utc_iso(), 'Rise' if yi else 'Set')
Arcminutes refraction for body seen at horizon: 34.53

2020-02-01T09:29:13Z Rise
2020-02-01T18:43:00Z Set

Adjusting for apparent radius

Planets and especially the Sun and Moon have an appreciable radius, and we usually consider the moment of sunrise to be the moment when its bright limb crests the horizon — not the later moment when its center finally rises into view. Set the parameter radius_degrees to the body’s apparent radius to generate an earlier rising and later setting; the value 0.25, for example, would be a rough estimate for the Sun or Moon.

The difference in rising time can be a minute or more:

f = almanac.risings_and_settings(eph, eph['Sun'], bluffton, radius_degrees=0.25)
t, y = almanac.find_discrete(t0, t1, f)
print(t[0].utc_iso(' '), 'Limb of the Sun crests the horizon')

f = almanac.risings_and_settings(eph, eph['Sun'], bluffton)
t, y = almanac.find_discrete(t0, t1, f)
print(t[0].utc_iso(' '), 'Center of the Sun reaches the horizon')
2020-02-01 12:46:27Z Limb of the Sun crests the horizon
2020-02-01 12:47:53Z Center of the Sun reaches the horizon

Elevated vantage points

Rising and setting predictions usually assume a flat local horizon that does not vary with elevation. Yes, Denver is the Mile High City, but it sees the sun rise against a local horizon that’s also a mile high. Since the city’s high altitude is matched by the high altitude of the terrain around it, the horizon winds up in the same place it would be for a city at sea level.

But sometimes you need to account not only for local elevation, but for altitude above the surrounding terrain. Some observatories, for example, are located on mountaintops that are much higher than the elevation of the terrain that forms their horizon. And Earth satellites can be hundreds of kilometers above the surface of the Earth that produces their sunrises and sunsets.

You can account for high altitude above the horizon’s terrain by setting an artificially negative value for horizon_degrees. If we consider the Earth to be approximately a sphere, then we can use a bit of trigonometry to estimate the position of the horizon for an observer at altitude:

from numpy import arccos
from skyfield.units import Angle

# When does the Sun rise in the ionosphere’s F-layer, 300km up?
altitude_m = 300e3

earth_radius_m = 6378136.6
side_over_hypotenuse = earth_radius_m / (earth_radius_m + altitude_m)
h = Angle(radians = -arccos(side_over_hypotenuse))
print('The horizon from 300km up is at %.2f degrees' % h.degrees)

f = almanac.risings_and_settings(
    eph, eph['Sun'], bluffton, horizon_degrees=h.degrees,
    radius_degrees=0.25,
)
t, y = almanac.find_discrete(t0, t1, f)
print(t[0].utc_iso(' '), 'Limb of the Sun crests the horizon')
The horizon from 300km up is at -17.24 degrees
2020-02-01 00:22:42Z Limb of the Sun crests the horizon

When writing code for this situation, we need to be very careful to keep straight the two different meanings of altitude.

  1. The altitude above sea level is a linear distance measured in meters between the ground and the location at which we want to compute rises and settings.
  2. The altitude of the horizon names a quite different measure. It’s an angle measured in degrees that is one of the two angles of the altitude-azimuth (“altazimuth”) system oriented around an observer on a planet’s surface. While azimuth measures horizontally around the horizon from north through east, south, and west, the altitude angle measures up towards the zenith (positive) and down towards the nadir (negative). The altitude is zero all along the great circle between zenith and nadir.

The problem of an elevated observer unfortunately involves both kinds of altitude at the same time: for each extra meter of “altitude” above the ground, there is a slight additional depression in the angular “altitude” of the horizon on the altazimuth globe.

When a right ascension and declination rises and sets

If you are interested in finding the times when a fixed point in the sky rises and sets, simply create a star object with the coordinates of the position you are interested in (see Stars and Distant Objects). Here, for example, are rising and setting times for the Galactic Center:

galactic_center = api.Star(ra_hours=(17, 45, 40.04),
                           dec_degrees=(-29, 0, 28.1))

f = almanac.risings_and_settings(eph, galactic_center, bluffton)
t, y = almanac.find_discrete(t0, t1, f)

for ti, yi in zip(t, y):
    verb = 'rises above' if yi else 'sets below'
    print(ti.utc_iso(' '), '- Galactic Center', verb, 'the horizon')
2020-02-01 10:29:00Z - Galactic Center rises above the horizon
2020-02-01 18:45:46Z - Galactic Center sets below the horizon

Solar terms

The solar terms are widely used in East Asian calendars.

from skyfield import almanac_east_asia as almanac_ea

t0 = ts.utc(2019, 12, 1)
t1 = ts.utc(2019, 12, 31)
t, tm = almanac.find_discrete(t0, t1, almanac_ea.solar_terms(eph))

for tmi, ti in zip(tm, t):
    print(tmi, almanac_ea.SOLAR_TERMS_ZHS[tmi], ti.utc_iso(' '))
17 大雪 2019-12-07 10:18:28Z
18 冬至 2019-12-22 04:19:26Z

The result t will be an array of times, and y will be integers in the range 0–23 which are each the index of a solar term. Localized names for the solar terms in different East Asia languages are provided as SOLAR_TERMS_JP for Japanese, SOLAR_TERMS_VN for Vietnamese, SOLAR_TERMS_ZHT for Traditional Chinese, and (as shown above) SOLAR_TERMS_ZHS for Simplified Chinese.

Lunar eclipses

Skyfield can find the dates of lunar eclipses.

from skyfield import eclipselib

t0 = ts.utc(2019, 1, 1)
t1 = ts.utc(2020, 1, 1)
t, y, details = eclipselib.lunar_eclipses(t0, t1, eph)

for ti, yi in zip(t, y):
    print(ti.utc_strftime('%Y-%m-%d %H:%M'),
          'y={}'.format(yi),
          eclipselib.LUNAR_ECLIPSES[yi])
2019-01-21 05:12 y=2 Total
2019-07-16 21:31 y=1 Partial

Note that any eclipse forecast is forced to make arbitrary distinctions when eclipses fall very close to the boundary between the categories “partial”, “penumbral”, and “total”. Skyfield searches for lunar eclipses using the techniques described in the Explanatory Supplement to the Astronomical Almanac.

To help you study each eclipse in greater detail, Skyfield returns a details dictionary of extra arrays that provide the dimensions of the Moon and Earth’s shadow at the height of the eclipse. The dictionary currently offers the following arrays, whose meanings are hopefully self-explanatory:

By combining these dimensions with the position of the Moon at the height of the eclipse (which you can generate using Skyfield’s usual approach to computing a position), you should be able to produce a detailed diagram of each eclipse.

For a review of the parameters that differ between eclipse forecasts, see NASA’s Enlargement of Earth’s shadows page on their Five Millennium Canon site. If you need lunar eclipse forecasts generated by a very specific set of parameters, try cutting and pasting Skyfield’s lunar_eclipses() function into your own code and making your adjustments there — you will have complete control of the outcome, and your application will be immune to any tweaking that takes place in Skyfield in the future if it’s found that Skyfield’s eclipse accuracy can become even better.