peter279k / astrotools

General Purpose Astronomy Library for PHP

Astrotools

Astrotools is a general purpose PHP library for astronomy.

Table of Contents

• Requirements
  • PHP 5
• Installation
  • PHP 5
• Features
• Why not use PHP's calendar extension?
• Usage Examples
  • Calculation of the Julian day from Date/Time
  • Calculation of Date/Time from Julian day
  • Calculation of sidereal time for a given Date/Time
    • Greenwich sidereal time
    • Local sidereal time
  • Time helper
    • Convert hours, minutes, and seconds to decimal time
    • Convert decimal time to hours, minutes, and seconds
    • Convert between time and hour angle
  • Date of Easter Calculation
  • Calculation of ΔT
• Todo
• References

Requirements

• PHP >= 7.0
• bcmath extension

PHP 5

It's possible to use the 0.0.x branch of Astrotools with PHP 5.4 - 5.6. For details see the next section.

Installation

composer require mjaschen/astrotools

PHP 5

If you need to use Astrotools with PHP 5.4, 5.5, or 5.6, just require version 0.0.2:

composer require mjaschen/astrotools:0.0.2

Please keep in mind, that the 0.0.x branch is no longer maintained and won't get any updates.

Features

• Julian day calculation (forward/reverse)
• Sidereal time calculation (GMST, local sidereal time)
• Date of Easter calculation
• Time helper

Why not use PHP's calendar extension?

Some of the implemented features (e.g. calculation of Julian day or Date of Easter) are already provided by PHP's calendar extension.

These functions come with some problems, e. g. easter_date() can only calculate the Date of Easter for the timerange of unix timestamps (1970 January 1 to somewhere around 2037/2038).

Usage Examples

Calculation of the Julian day from Date/Time

use Astrotools\Time\JulianDay;

$timestamp = new \DateTime('2015-02-25 12:01:36', new \DateTimeZone('Europe/Berlin'));
$jd = new JulianDay($timestamp);

echo $jd->getValue();

The code above produces the output shown below:

2457078.9594444

Calculation of Date/Time from Julian day

use Astrotools\Time\JulianDay;

$jd = new JulianDay();
$jd->setValue(2451545.0);

var_dump($jd->getDateTime());

The code above produces the output shown below:

class DateTime#10 (3) {
  public $date =>
  string(26) "2000-01-01 12:00:00.000000"
  public $timezone_type =>
  int(3)
  public $timezone =>
  string(3) "UTC"
}

Calculation of sidereal time for a given Date/Time

Greenwich sidereal time

use Astrotools\Time\SiderealTime;

$dt = new \DateTime('2007-12-25 00:00:00', new \DateTimeZone('UTC'));
$st = new SiderealTime($dt);

echo $st->getSiderealTime();

The code above produces the output shown below:

6.2086583333

Local sidereal time

Local sidereal time for Berlin, Germany (longitude = 13.5 degrees east):

use Astrotools\Time\SiderealTime;

$dt = new \DateTime('2007-12-25 20:00:00', new \DateTimeZone('UTC'));
$st = new SiderealTime($dt);

echo $st->getLocalSiderealTime(13.5);

The code above produces the output shown below:

3.1634161794

Time helper

Convert hours, minutes, and seconds to decimal time

use Astrotools\Helper\Time;

$time = new Time(6, 42, 23.1337);

echo $time->getValue();

The code above produces the output shown below:

6.7064260278

Convert decimal time to hours, minutes, and seconds

use Astrotools\Helper\Time;

$time = new Time(6.7064260278);

printf('%02d:%02d:%02.4f', $time->getHour(), $time->getMinute(), $time->getSecond());

The code above produces the output shown below:

06:42:23.1337

Convert between time and hour angle

use Astrotools\Helper\Time;

$time = new Time(6, 42, 23.1337);

echo $time->getHourAngle() . PHP_EOL;

$time->setHourAngle(275);

echo $time->getValue();

The code above produces the output shown below:

100.596390417
18.333333333333

Date of Easter Calculation

use Astrotools\Time\DateOfEaster;

$doe = new DateOfEaster(2000);

echo $doe->getDate()->format('Y-m-d');

The code above produces the output shown below:

2000-04-23

Calculation of ΔT

There exist multiple methods to get the value of ΔT for a given (decimal) year. One can lookup the value in tables and interpolate it for the wanted date or calculate it using polynomial expressions.

Astrotools currently provides the calculation of ΔT with polynomial expressions. Reasonably accurate values are provided for the timespan between the years -500 and 2150.

<?php

use Astrotools\Time\DeltaT\PolynomialExpression;

$deltaT = new PolynomialExpression();

echo $deltaT->getDeltaT(2016.125);

The code above produces the output shown below:

69.568218578125

Todo

• Support for microseconds in Julian day calculation
• Implement ΔT determination by table lookups

References

• Julian day formula - Astronomical Applications Department of the U.S. Naval Observatory
• Julian Date Converter - Astronomical Applications Department of the U.S. Naval Observatory
• Julian day - Wikipedia
• ΔT
• Astronomical Algorithms, Jean Meuss, 1998, ISBN 978-0943396613
• Polynomial Expressions For Delta T (ΔT)