Content Questions
Instead of star names, how does one get to list the sky by
constellations instead?
In the SkyGazer ``Display'' menu, choose ``Constellations'', and then pick the subset you want. You can also choose whether it should display constellation area boundaries, or the ``stick figures''. (One annoying thing: in this software version, it seems hard to turn the constellations off once they are displayed. It might be a bug. If anyone figures out how to do this, let me know!)
You can also click on an area of the sky to make it display information
about the constellation corresponding to that region of the sky.
I'm still confused about where your horizon is on the sky
chart
When you use a sky chart (on SkyGazer or a paper chart), you
orient yourself facing some particular direction (say, north), and then
orient the chart in front of you such that the lower edge
of the circle labeled north is down (on SkyGazer, you click the
buttons to rotate; if your chart is on a piece of paper,
you just rotate the paper). This lower edge of the circle
represents the horizon you are facing.
Then if you raise the chart
over your head, facing you, the chart represents the sky that
you see. The objects shown above the curved part of the circular sky chart
will be seen above the horizon you are facing. The center of
the circular chart is the zenith above you.
If you look in another direction, just rotate the chart so that
the bottom of the circle points in that direction,
and look above that horizon.
Where exactly are the equinoxes and solstices in the sky?
The equinoxes are the points on the celestial sphere where
the celestial equator intersects the ecliptic plane: for the spring
equinox, this is just below the constellation Pisces, and for the fall equinox
it's on the opposite side of the sky, below Virgo. The solstices
are the points on the celestial sphere where the Sun is when the Earth
is tipped either completely away or towards the Sun. The winter
solstice is located in Sagittarius, and the summer sostice in Gemini.
There don't happen to be any bright stars near these points.
So the solstices/equinoxes are points in the sky... when the
moment occurs is when the Sun is over them?
Right. For instance, at the time of spring equinox, the Sun is right
on top of the spring equinox, so you can't see that point in the
sky (or points near it).
Can you explain how the Sun follows the ecliptic and also
the analemma?
One of these is a path in the celestial sphere, and the other is a path in the local sky.
The ecliptic is the path of the Sun on the celestial sphere, the imaginary sphere that rotates around the Earth once per day. The Sun is of course going around the local sky every day along with the celestial sphere, but it also slowly moves around the celestial sphere, once per year, because of the orbit of the Earth around the Sun.
The analemma is another thing altogether (we didn't discuss it in
class). This describes the figure 8 shape
you get when you trace the
position of the Sun at the same time each day in the local
sky. Note that the celestial sphere looks different at
the same time of day, at different times of the year!
Why is the celestial coordinate system used?
The celestial coordinate system is very useful for specifying
locations of astronomical objects that can be considered fixed on the
celestial sphere. Note that local altitude/azimuth coordinates are
handy for pointing out the location of some object in the local sky
now, but if the object is a star or other distant object,
that object will have rotated around and be somewhere else a bit
later! If you want to tell your friend where Vega is by giving them
local altitude/azimuth coordinates, you'll have to tell them what time
and what place on Earth those coordinates are relevant for, and that
person will have to do some calculations to figure out where to look
at his or her current location and time. From celestial coordinates,
on the other hand, you can figure out where the object is at any time
for your position.
I'm confused about RA and dec.
These represent coordinates of an object on the celestial sphere.
Declination is like latitude on the celestial sphere; right ascension
is like longitude. Declination is defined
with respect to the celestial equator, and right ascension with respect
to the ``longitude'' line that goes through the spring equinox.
I will go over these again next class.
How do you find the Sun's coordinates for the solstices
and equinoxes?
The Sun goes around the ecliptic, which is at an angle of 23.5
from the celestial equator, due to the tilt of the Earth's axis: see
Figure S1.8 of your text, which was also shown in class. At the
solstices, you know that the Sun is at its highest (most northern) or
lowest (most southern) points away from the celestial equator. So,
for instance, at the summer solstice, declination must be 23.5
;
at the winter solstice, declination must be -23.5; at the
equinoxes, since the Sun is right on the celestial equator,
declination must be 0. Right ascension is determined from the
angular distance around the north pole, starting at the spring
equinox. So for the spring equinox, the Sun's RA is 0h. A quarter of
the way around, it's the summer solstice: RA is one quarter of 24
hours, or 6h. Halfway around, it's the fall equinox: RA is 12h. The
winter solstice has RA 18h.
I didn't understand the PRS question
See above question and Figure S1.12 of the text: at the winter solstice,
the Sun is at minimum declination, and it's three quarters of the way
around from the spring equinox, so it has RA of 18h.
What are the most distant things that orbit our Sun?
There's a cloud of icy, comet-like objects called the ``Oort cloud'',
extending to tens of thousands of AU from our Sun. Comets are thought
to come from this cloud. We'll talk more about this later.