Astronomy

Astronomy

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I started writing my own astronomy apps for my Timex Datalink USB after buying a telescope a few years ago... I normally like to use a shareware type program, Pocket Stars, on my PocketPC. It is very good when out in the field and need to quickly locate stuff. But, I wrote a few little apps myself for fun, and to understand astronomy better... I've found that there's a book that gives code for the calculations required. There's a GNU project, LIBNOVA, that has put all the calculations on the web. I used this in one of my apps...

Calculating the location of a star in the sky involves knowing the position of that star in the celestial sphere, your location on earth, and the time. Conversely, you can use your observation of a stars location in the sky to calculate your location on earth or the time. For this reason, astronomy was very important to ancient mariners, who needed to know their location.

Terminology:

Epoch This is the time when an observation was made. The stars are pretty much fixed in the sky, but not exactly... If you were to track the location (RA and DEC) of a star over many years, you would find that it slowly moves. This is mostly due to the precession of earth's orbit around the sun, but also due to the sun's actual movement relative to other stars. So, when the RA and DEC of a star is specified, the epoch, or time when they were measured is also specified. An epoch seems to be good for 40 years or so... The currently used epoch is J2000.0.

JD The Julian Date is commonly used to specify the time in calculations. It is the number of days since noon GMT on Monday, January 1, 4713 BC. The integer form is called the Julian Day. As of this writing, the JD is 2454091.44792. The Julian date of the current epoch, J2000.0, is exactly 2451545.0.

Celestial Sphere Since stars are so far away, their actual distance doesn't matter so much. So, the stars can all be imagined to lie on the surface of a big sphere, the celestial sphere. The line between Earth's poles is extended to this sphere to define a north and south pole. The Earth's equator is projected to define the celestial equator. In this way, an equivalent latitude of a star can be defined. Defining an equivalent longitude of a star is more complex since the Earth is spinning and the celestial sphere is not. In the equatorial coordinate system, the vernal equinox is used as a basis for this.

Geographic Coordinates The usual way to give your position on earth is through your longitude and latitude.

Equatorial Coordinates Most common way to specify the locations of stars and other bodies in the celestial sphere. This is basically a projection of earth's system of longitude and latitude onto the celestial sphere at the instant of the vernal equinox. But, instead of longitude and latitude, the terms are Right Ascension (α or RA) and Declination (δ or DEC). RA is like longitude and can be expressed in terms of hours or degrees. DEC is like latitude with 0 corresponding to the equator and +90 or North 90, the north pole, and -90 or South 90, the south pole. Because the RA and DEC of a star slowly changes over time, the epoch of the coordinates must be specified.

Vernal Equinox The time in spring, ~ March 20th, when the days and nights are equal length. More precisely, it is the exact time when the sun is directly on the celestial equator. There are actually two places where the celestial equator and the plane of the ecliptic cross. The other place is the Autumnal Equinox that occurs ~September 23. The vernal equinox is important in astronomy because it defines the equatorial coordinate system. Midway between equinoxes are the solstices (Winter and Summer) when the days are either longest or shortest. The vernal equinox is also called the "first point of Aires" because it also used the mark the beginning of the zodiac calendar where the sun entered the constellation Aires. But, because of the precession of the equinoxes, this point is now in the constellation Pisces. By definition, the first point of Aires is located at RA=DEC=0.

Horizontal coordinate system Most common way to specify the apparent location of stars and other bodies. Coordinates are altitude, Alt, (or elevation) and azimuth, Az. Altitude is the angle of the object above the horizon. Straight up is 90 degrees and called zenith. Sometimes the zenith distance, Zd, (really an angle) is used instead, which is the angle down from the zenith to the object so that Alt=90-Zd. Azimuth is the angle between the object and the north pole. So east is 90 degrees, south is 180 degrees, and west is 270 degrees.

Local Mean Sidereal Time or LST indicates your longitudinal position on earth relative to the celestial sphere. When expressed as hours, it is equal to how many hours have passed since the vernal equinox point was directly overhead, on your local meridian (line on celestial sphere passing from north pole to zenith to south pole). Sidereal time is different than local time by one day a year because the earth is rotating about the sun.

Hour Angle The hour angle (HA) along with declination can be used to specify the apparent position of a star. The hour angle indicates how many hours have passed since the star was on your local meridian (line on celestial sphere passing from north pole to zenith to south pole). HA=LST-RA. So, your LST is equal to the HA of the vernal equinox (where RA=0).

UTC (formerly GMT) or Coordinated Universal Time is the official time in Greenwich, England and is the time normally used by computers. Your local time is UTC plus your time zone offset plus daylight savings time offset. UTC is kept by atomic clocks, so seconds are SI standard seconds, but there are leap second adjustments to keep 365.252460*60 seconds equal to the apparent solar year. These adjustments are necessary because the Earth's speed around the sun is slowing every year. Daylight savings are not applied to UTC.

UT1 has a variable length second so that there are always 86,400 seconds in a day. UTC is kept close to UT1 by the application of leap seconds. UTC=UT1+sum of applied leap seconds.

TAI or International Atomic Time is the time kept by a system of atomic clocks at sea level. The second is the SI second and there are no leap adjustments. UT1 was equal to TAI on January 1, 1958.

TT or Terrestrial Time = TAI + 32.184s

Daylight Savings Time or Summer Time is a 1 hour adjustment made to local time between spring and fall to save energy. This can be tricky to deal with because it's application is non-uniform. Fortunately, most computer systems handle this automatically and can give you the UTC time.