In a typical coordinate system you have at least two reference planes and then any location
can be explained as the distance from those planes.
So, for example, for latitude/longitude, the two reference planes are the equator and the
prime meridian in Greenwich, england.
Latitude is your angular distance either north or south from the equator and longitude is
your angular distance east or west from Greenwich.
So when I say my coordinates right now are 42 degrees north by 71 degrees west.
That means I’m 42 degrees north of the equator, and 71 degrees west of the British Royal Observatory
in Greenwich, England.
Now you may have never considered why do we start measuring Longitude from that specific
place in Greenwich.
I won’t answer that in this video, but I will point you to a great book by Dava Sobel
called Longitude for the whole story there.
Now, let’s move on to the stars in what we call a celestial coordinate system.
There are lots of possible reference planes for making a celestial coordinate system and
one of the earliest used by ancient people was the ecliptic which is the path that the
sun moves throughout the year and is where the zodiac constellations come from.
The more common coordinate system today though is equatorial which as the name suggests uses
the earth’s equator as a reference plane, basically just imagine the equator of the
earth going out into space forever.
The other reference plane is a bit more complex to wrap your head around as it’s both a
time and a location.
The reason it has to be both a time and a location is we are both rotating around our
own axis, but also rotating around the sun, which is why there are different constellations
up in the summer compared to the winter, because we are literally facing a different part of
the milky way.
So the other reference plane is where the sun is located in the night sky at the time
of the vernal equinox in March, we then measure where the stars are located eastward from
that plane and call this right ascension.
So to review: the two parts that make up equatorial coordinates for the night sky are first right
ascension which is measured eastward from where the sun is in the night sky on the vernal
equinox, and then declination which is measured from the earth’s equator projected out into
space, up or down, with the equator being 0 degrees.
Now to make this just a bit more confusing.
Declination like latitude is measured in degrees.
Right ascension could be measured in degrees and sometimes is, but more commonly is measured
in hours, minutes and seconds.
So instead of breaking the celestial sphere into 360 parts we call degrees, we break it
into 24 parts we call hours like a clock.
Okay, so that’s the equatorial coordinate system, you can use a star chart and what
are called setting circles on an equatorial mount if you want to go full manual.
What I think of as the ‘GPS’ of equatorial mounts is a GO-TO system where you tell the
system where on earth you are located and the precise time and date, and then it translates
where the stars should be in the sky, and automates the pointing of the telescope.
Now the last thing I want to talk about is another kind of coordinate system we use all
the time in astronomy called a horizontal coordinate system, also known simply as Alt-Az.
Which is short for altitude and azimuth.
With alt-az the two reference planes are North and the horizon at the actual time and location
of where you are observing.
It’s called a horizontal coordinate system, but another way I’d think of it is just
a local coordinate system that’s just for that moment.
The coordinates don’t really mean anything outside of that context.
You can see if I change my time of day in stellarium, the alt/az keep changing, but
the Ra/dec stay consistent.
BUT the reason we use alt/az all the time is they are a lot more easy to understand.
For instance if I pull up Alberio here and see the azimuth is 90 degrees I know that
is due east, and if it’s altitude is 45 degrees I know that’s halfway up from the
horizon to the zenith.
The Zenith means straight up, which is 90 degrees.
Now there are some fancy formulas for converting equatorial coordinates to horizontal coordinates,
but luckily we live in an age where all of that info is included in free software like
stellarium or any number of free or low cost planetarium apps on your smartphone.
Next week I’ll be talking about the apparent size of objects in space, and how calculating
your field of view can tell you what kinds of deep sky objects will work best for your
gear.
Till then, this has been Nico Carver from Nebula Photos.
Clear skies!