## Index of all namespaces

Library functions based on the book Astronomical Algorithms

Protocol, classes and functions for celestial bodies References: [1] Meeus, Jean. _Astronomical Algorithms_ 2nd Ed. Willman-Bell, Inc. 1998.

Functions to calculate local coordinates and passages References: [1] Meeus, Jean. _Astronomical Algorithms_ 2nd Ed. Willman-Bell, Inc. 1998.

Functions for unit conversions References: [1] Meeus, Jean. _Astronomical Algorithms_ 2nd Ed. Willman-Bell, Inc. 1998.

Functions for converting dates and times relevant to astronomical computation References: [1] Meeus, Jean. _Astronomical Algorithms_ 2nd Ed. Willman-Bell, Inc. 1998.

Get historical and extrapolated values for Delta T

# astro-algo

## A Clojure library designed to implement computational methods described in Jean Meeus’ *Astronomical Algorithms*.

The astro-algo library provides a celestial Body protocol with interfaces for equatorial coordinates and standard altitude. A single implementation of the Body protocol is provided for the Sun. The library also provides a public function to get local coordinates for a Body at any time and place and a public function to get passages (rising, transit, setting) for a Body on any day at any location.

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.

The library also includes date utils with a reduction of time scales table that spans from 1890 to 2020.

## Usage

```
(:require
[com.climate.astro-algo.bodies :refer [local-coordinates passages]]
[clj-time.core :as ct])
(:import
[com.climate.astro_algo.bodies Sun])
(let [sun (Sun.)]
; get cosine angle of the sun
; on 10/18/1989 00:04:00 (UTC) in San Francisco
(let [dt (ct/date-time 1989 10 18 0 4 0)
lon -122.42 ; [degrees east of Greenwich]
lat 37.77 ; [degrees north of the Equator]
; get azimuth and altitude in radians
{:keys [azimuth altitude]} (local-coordinates sun dt lon lat)]
; get the cosine of the zenith (the sin of the altitude)
; negative value indicates the sun is geometrically beneath the horizon
(Math/sin altitude)))
(let [sun (Sun.)]
; get civil daylength on 12/21/2013 at Stonehenge
(let [date (ct/local-date 2012 12 21)
lon -1.8262 ; [degrees east of Greenwich]
lat 51.1788 ; [degrees north of the Equator]
{:keys [rising transit setting]} (passages sun date lon lat
; can be none, civil, astronomical, nautical
; none is default
:include-twilight "civil"
; set lower precision (degrees)
; to speed up computation
; 0.1 is default
:precision 1.0)]
; calculate daylength in hours
(-> (ct/interval rising setting)
ct/in-millis ; [milliseconds]
(/ 3600000) ; [hours]
double)))
```

## License

Copyright © 2017 The Climate Corporation. Distributed under the Apache License, Version 2.0. You may not use this library except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

See the NOTICE file distributed with this work for additional information regarding copyright ownership. Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.