0:02 let's talk now about a way that we can
0:05 define any location in the sky using a
0:07 unique set of coordinates to do that
0:10 let's first use an analogy we know from
0:13 earth that we can take our own rotation
0:15 axis and use that to divide the earth
0:18 along the equator into northern and
0:20 southern hemispheres that also means
0:22 that we can now draw parallel lines to
0:24 the equator giving us what we call
0:26 parallels of latitude so we start at
0:28 zero degrees at the equator and we
0:30 arrive at 90 degrees north at the North
0:32 Pole and 90 degrees south of the South
0:35 Pole now in addition to parallels of
0:37 latitude measuring north and south we
0:39 also want to be able to measure from
0:41 east to west so by international
0:44 agreement the prime meridian is an
0:46 imaginary line that goes through the
0:48 North Pole through Greenwich England at
0:51 the Royal Observatory all the way down
0:53 to the South Pole and then we can simply
0:55 measure east or west in terms of
0:58 meridians of longitude so for example
1:00 here in Baltimore we are at 39 degrees
1:04 north latitude 76 degrees west latitude
1:06 a southern example would be cerro
1:08 paranal and chile that's 24 degrees
1:10 south latitude or 70 degrees west and
1:14 rome italy is 42 degrees north and 12
1:16 degrees east so that places rome at
1:19 about the same latitude as boston
1:22 massachusetts now that is how we define
1:25 every location on earth and we're going
1:28 to use an analogous system to define
1:31 every location in the heavens so let's
1:33 bring our earth inside of the celestial
1:36 sphere and will once again extend our
1:39 north and south poles to form the north
1:41 and south celestial poles we'll extend
1:43 the equator to form the celestial
1:45 equator and just as we did before with
1:48 parallels of latitude we can now draw
1:50 parallel lines to the celestial equator
1:53 only we refer to these as parallels of
1:57 declination so we measure declination as
2:00 zero degrees from the equator all the
2:02 way up to positive 90 at the North
2:04 celestial Pole and then all the way down
2:07 to negative 90 at the South celestial
2:10 Pole now we cannot simply take our
2:13 meridians and apply those to the sky as
2:13 well the reason
2:15 for that is because the earth is
2:17 rotating and therefore the meridians
2:19 would need to rotate as well and that
2:21 would make such a system fairly useless
2:24 to us instead what we'll do is we'll
2:26 take the annual path of the Sun the
2:29 ecliptic and we'll note the location
2:31 that the Sun is on in March when it
2:33 arrives at the vernal equinox
2:36 since the Earth rotates on its axis once
2:37 every 24 hours
2:41 this gives us a 24-hour clock face that
2:44 we can write on to the celestial equator
2:47 so when we draw a parallel lines to this
2:50 clock face we then get hours of right
2:53 ascension so again think of not so much
2:55 as hours of time but think of it instead
2:59 as hours on a clock face for example
3:02 Rigel in the constellation of Orion has
3:04 a right ascension of a little more than
3:07 five hours about a quarter of the way on
3:10 to the sixth hour circle so that gives
3:11 Rigel a right ascension of five hours
3:14 and 15 minutes and since it's south of
3:16 the celestial equator that gives us a
3:19 declination of minus eight degrees and
3:23 twelve minutes of Arc remember we can
3:26 take a single degree and we can split
3:31 that up into 60 minutes of Arc so this
3:33 coordinate system since it's based on
3:35 the celestial equator we call this the
3:38 equatorial coordinate system and it's a
3:41 really convenient way for us to define
3:44 every single point on the celestial sphere
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