5.3 Equatorial Coordinate System
57
the vernal equinox (also known as the first point of Aries, which for reasons that
will become clear is no longer in Aries) and the north and south celestial poles to
determine the origin.
The equatorial system identifies an object by its right ascension (R A or α), which
is the angular distance eastwards along the celestial equator between the object and
the first point of Aries, and its declination (Dec or δ), which is the angular separation
between the object and the celestial equator. Objects lying on the celestial equator
have a declination of 0
◦ , with those north and south of the line having positive
and negative values respectively. Typically, right ascension is measured in hours,
minutes, and seconds, and declination in degrees, minutes, and seconds, that is, in
sexagesimal notation. You should be aware that minutes and seconds in RA are not
of the same length as minutes and seconds in Dec. If you are performing positional
measurements, you may wish to to resolve this problem by converting your RA and
Dec to decimal notation.
You should be aware that on occasion, decimal degrees may be required or supplied. In general, decimal degrees are given as just a decimal number, while sexagesimal notation has the individual components separated by a space, by hms (for
hours, minutes, and seconds), or more commonly, by a colon. Hence the star known
as Betelgeuse, or α Orionis, has an RA and Dec denoted by 05 h 55 m 10 s +07 d
24 m 25 s or 05:55:10+07:24:25 or, in decimal notation, 238.7917+7.406944.
The Earth’s axis is precessing, like a slowing spinning top. Consequently, over
long time periods, points that seem fixed gradually reveal themselves to be in motion.
For example, the celestial poles appear to describe circles. It is for this reason
1 that
the first point of Aries is no longer is Aries and also why there are now 13 zodiacal
constellations. Over short periods, nutation does not pose a problem for astronomers.
However, over longer periods stars will move away from their catalogue positions
due to the rocking of the Earth’s axis, a process known as nutation. In an attempt
to resolve this problem, astronomical catalogues have fixed dates for the positions
they represent, known as the epoch. The current epoch is J2000; it uses positions
determined from midday 1 January 2000. The previous epoch was B1950, which was
in use prior to 1984. Although you should encounter only J2000 coordinates, if you
are using an older catalogue or academic paper, you may come across B1950 or even
older epochal systems. In such cases, you should endeavour to find the object’s J2000
coordinates, using, for example, the SIMBAD astronomical catalogue. You may also
encounter JNow when you are using astronomical software, which gives an object’s
RA and Dec based on its J2000 positions, the current date, and the precision rate.
In general, users should avoid using JNow, as at the time of writing the differences
between J2000 and JNow are minimal. However, as J2000 becomes increasingly
outdated, JNow may become necessary (Fig. 5.3).
For an observer in the northern hemisphere, stars with declinations greater than
θ − 90
◦ (where θ is the observer’s latitude) should be visible at some point during
the year, although that depends on the quality of the local horizon. Likewise, in the
southern hemisphere, stars with declinations less than θ + 90
◦ will be visible.
1 As well as others that are beyond the scope of this book.
57
the vernal equinox (also known as the first point of Aries, which for reasons that
will become clear is no longer in Aries) and the north and south celestial poles to
determine the origin.
The equatorial system identifies an object by its right ascension (R A or α), which
is the angular distance eastwards along the celestial equator between the object and
the first point of Aries, and its declination (Dec or δ), which is the angular separation
between the object and the celestial equator. Objects lying on the celestial equator
have a declination of 0
◦ , with those north and south of the line having positive
and negative values respectively. Typically, right ascension is measured in hours,
minutes, and seconds, and declination in degrees, minutes, and seconds, that is, in
sexagesimal notation. You should be aware that minutes and seconds in RA are not
of the same length as minutes and seconds in Dec. If you are performing positional
measurements, you may wish to to resolve this problem by converting your RA and
Dec to decimal notation.
You should be aware that on occasion, decimal degrees may be required or supplied. In general, decimal degrees are given as just a decimal number, while sexagesimal notation has the individual components separated by a space, by hms (for
hours, minutes, and seconds), or more commonly, by a colon. Hence the star known
as Betelgeuse, or α Orionis, has an RA and Dec denoted by 05 h 55 m 10 s +07 d
24 m 25 s or 05:55:10+07:24:25 or, in decimal notation, 238.7917+7.406944.
The Earth’s axis is precessing, like a slowing spinning top. Consequently, over
long time periods, points that seem fixed gradually reveal themselves to be in motion.
For example, the celestial poles appear to describe circles. It is for this reason
1 that
the first point of Aries is no longer is Aries and also why there are now 13 zodiacal
constellations. Over short periods, nutation does not pose a problem for astronomers.
However, over longer periods stars will move away from their catalogue positions
due to the rocking of the Earth’s axis, a process known as nutation. In an attempt
to resolve this problem, astronomical catalogues have fixed dates for the positions
they represent, known as the epoch. The current epoch is J2000; it uses positions
determined from midday 1 January 2000. The previous epoch was B1950, which was
in use prior to 1984. Although you should encounter only J2000 coordinates, if you
are using an older catalogue or academic paper, you may come across B1950 or even
older epochal systems. In such cases, you should endeavour to find the object’s J2000
coordinates, using, for example, the SIMBAD astronomical catalogue. You may also
encounter JNow when you are using astronomical software, which gives an object’s
RA and Dec based on its J2000 positions, the current date, and the precision rate.
In general, users should avoid using JNow, as at the time of writing the differences
between J2000 and JNow are minimal. However, as J2000 becomes increasingly
outdated, JNow may become necessary (Fig. 5.3).
For an observer in the northern hemisphere, stars with declinations greater than
θ − 90
◦ (where θ is the observer’s latitude) should be visible at some point during
the year, although that depends on the quality of the local horizon. Likewise, in the
southern hemisphere, stars with declinations less than θ + 90
◦ will be visible.
1 As well as others that are beyond the scope of this book.