Introduction to astro-algo

Astro-Algo is a Clojure library designed to implement computational methods described in Astronomical Algorithms by Jean Meeus. In this release, the following is complete:

• date utils with reduction of time scales table from 1890 to 2016
• celestial Body protocol to get equatorial coordinates and standard altitude
• implementation of Body protocol for the Sun
• function to get local coordinates for a body
• function to get passages (rising, transit, setting) for a body

The solar position computations implemented in this library are considered low accuracy. They account for some effects of nutation with simplifications described in AA Ch. 25. These calculations should be accurate to within 0.01 degrees of /geometric position/, however larger differences may be observed due to atmospheric refraction, which is not accounted for other than the standard values for the effect of atmospheric refraction at the horizon used in the calculation of passage times.

Passages are calculated according to AA Ch. 15 by interpolating the time that the celestial body crosses the local meridian (for transit) or the standard altitude (for rising and setting). Standard altitudes for rising and setting are taken from the celestial body unless the passages for twilight are desired, in which case the standard altitude for the specified twilight definition is used.

API

com.climate.astro-algo.bodies

protocol Body

equatorial-coordinates

Calculate apparent right ascension and declination of a body.
Input:
  this (Body instance)
  dt (org.joda.time.DateTime UTC)
Returns a map with keys:
  :right-ascension (radians west of Greenwich)
  :declination (radians north of the celestial equator)

standard-altitude

Returns the "standard" altitude, i.e. the geometric altitude of the
center of the body at the time of apparent rising or setting
Input:
  this (Body instance)
Returns standard altitude (degrees)

local-coordinates

Calculate local coordinates of a celestial body
Input:
  body (Body instance)
  dt (org.joda.time.DateTime UTC)
  lon - longitude (degrees East of Greenwich)
  lat - latitude (degrees North of the Equator)
Returns a map with keys:
  :azimuth (radians westward from the South)
  :altitude (radians above the horizon)

passages

Calculate the UTC times of rising, transit and setting of a celestial body
Input:
  body (Body instance)
  date (org.joda.time.LocalDate)
  lon - longitude (degrees East of Greenwich)
  lat - latitude (degrees North of the Equator)
  opts
    :include-twilight (one of "civil" "astronomical" "nautical" default nil)
    :precision - iterate until corrections are below this (degrees, default 0.1)
Returns a map with keys:
  :rising (org.joda.time.DateTime UTC)
  :transit (org.joda.time.DateTime UTC)
  :setting (org.joda.time.DateTime UTC)

com.climate.astro-algo.compute

nutation-in-longitude

Calculate the longitude of the ascending node of the Moon's orbit
on the ecliptic, measured from the mean equinox of the date
Input: t (centuries since J2000.0)
Returns: longitude (radians)

nutation-in-obliquity

Calculate the nutation in obliquity of the ecliptic
Input: t (centuries since J2000.0)
Returns: longitude (degrees)

true-obliquity

Calculate the true obliquity of the ecliptic
Input: t (centuries since J2000.0)
Returns: true obliquity (degrees)

mean-sidereal-time

Calculate mean sidereal time at Greenwich
Input: dt (org.joda.time.DateTime)
Returns: sidereal time (degrees)

apparent-sidereal-time

Calculate apparent sidereal time at Greenwich
Input: dt (org.joda.time.DateTime)
Returns: sidereal time (degrees)

com.climate.astro-algo.conversions

deg->rad

Convert degrees to radians

rad->deg

Convert radians to degrees

instant->deg

Convert hours, minutes, seconds to degrees

angle->deg

Convert degrees, arcminutes, arcseconds to degrees

com.climate.astro-algo.date-utils

td->ut

Convert date string or org.joda.time.DateTime to Dynamical Time

ut->td

Convert Dynamical Time String or org.joda.time.DateTime to UTC

day-of-year

Get integer day of year from org.joda.time.DateTime
[1] Ch.7

decimal-day

Get decimal day of month from org.joda.time.DateTime

julian-day

Convert date string or org.joda.time.DateTime to Julian Day
[1] Eqn (7.1)

centuries-since-j2000

Convert date string or org.joda.time.DateTime to centuries since J2000.0
[1] Eqn (12.1)

com.climate.astro-algo.delta-t

delta-t

Get difference between dynamical time and universal time (delta_T = TD - UT)
Input: org.joda.time.DateTime
Returns: org.joda.time.Period