This exercise is to observe, in teams, a group of non-stellar objects. It is divided into two parts: planning the observation and carrying it out.
The position of an object in the sky is usually described by its coordinates in right ascension and declination. They are the celestial analogs of terrestrial longitude and latitude. Just as Honolulu and Seattle have fixed, known locations in longitude and latitude, so do Sirius and Aldebaran have fixed locations in right ascension (RA) and declination (dec). And just as when you fly in an airplane you can see some range of longitude and latitude at any one time, your view of the celestial sphere depends on the earth's rotation and position in its orbit.
Although the stars and other objects outside our solar system
have fixed positions on the RA-dec grid (well, they do move,
but slowly enough that it doesn't matter for casual observing),
the Sun's right ascension and declination change gradually during the year,
just due to the earth's orbital motion. The planets and
other solar system objects also appear to move through the
sky during the year.
This chart plots, for the Sun and the planets, each object's RA as a function of date during the year. The horizontal axis is date, with tick marks at the begining of each month. The vertical axis is the RA value. Right ascension is given in hours and minutes instead of degrees, which seems strange until you know that one hour in RA is equal to 15° on the sky, that is, the angle the sky appears to move in one hour. The Sun's position is plotted as a dark line on the chart; it starts the year near 19h RA and increases during the year. At the spring equinox the sun crosses the line at 24h, which is the same as 0h RA.
The planet positions are also plotted on the chart, so you can tell what part of the sky they occupy on a given date. Note that the inner planets are always near the sun, as we would expect. The outer planets are quite a bit farther away, so their position with respect to the star background only changes slowly.
Right ascension increases to the east, so after sunset, objects with larger RA than the sun will be visible. The area labeled Evening Sky is above the horizon at sunset; the area labeled Morning Sky is above the horizon at sunrise. The gray band on the chart indicates twilight; objects within this band will not be visible because they set too soon after the sun. So from the end of twilight, we can go about ten hours up (greater RA) and expect to find objects. Determine first what range of RA you can consider when selecting objects to view. Start by locating the date of the planned observation on the horizontal axis, then draw a vertical line on the chart at this date. The part of this line that's within the Evening Sky range indicates the range of RA that will be visible (except the part within about two hours of the sun).
The declination range available to us doesn't change during the year, but it's limited by our location in latitude. Since we are at latitude +21 degrees, we will never be able to see objects south of declination -70, and we know that objects too near the horizon will be difficult.
So, an object must be within the visible RA range and between about -60° and +90° declination to be considered for inclusion in your target list.
Next, where is the Moon? New moon occurs at about noon local time on February 8, 2005. What is its phase on the planned observing day, and when does it rise or set? This is important, because if the moon is out it makes the sky so bright that it's hopeless to locate faint objects.
Are any of the objects likely to be easier to locate than the rest? You need to decide whether to do these first, or start with the hard ones. Also, what if you don't get to all of them during the planned session? We can try again next week, but what about the moon on that day? If it will be bright, perhaps working on the fainter objects first is smart.
The telescope field of view is, as we know, determined by the eyepiece chosen. The eyepieces have an apparent field of about 50 degrees diameter, but this is reduced by the magnification to a field of 50/m degrees, where m = 1200/fl, and fl is the eyepiece focal length. It's useful to have a loop of wire, or a circular overlay of some kind, with the size of the finder field, and another with the field of one of the eyepieces.
You can work out your group strategy for making the finder charts. One option is that each member provide the charts for one of the objects. The web site Your Sky from Honolulu may be very useful for this, since you can adjust the scale.
Part of the observing plan is to plan how to get the telescope pointed at the target object, starting with known stars and moving from one star to another (it's called star-hopping) until you're pointed at the right place, even if you can't see it in the finder. Read the article in Sky and Telescope called Using a Map at the Telescope for a good description of how star-hopping can be used to systematically move the telescope to a desired location using a finding chart.
For each object, make a detailed plan of which stars will be used to hop from a known location to the position of the desired object. Some plans may be very simple, such as when you already know just where the object is and it's so bright you can't miss it. Others should be much more detailed, or you'll end up wasting lots of time with the telescope pointed the wrong direction.
The report should be a full report, describing the goals of the project, the methods and results. Include finder charts for one of the objects; then for each target object describe the observation process, and include your sketch and comments. Please comment on observing conditions.
Last modified: February 4, 2005
http://www.ifa.hawaii.edu/users/mickey/ASTR110L_S05/DarkSky.html