29:52 Exploration of the Solar System

            September 4, 2004                                                                                     

.           Addendum 1:  Notes on the SC1 Chart

 

One of your fellow students, Wayne Fauchier,  sent me an email asking some very good questions about the SC1 chart,  and the correspondence between it and what we see on the sky.  Since learning this is a major goal of this course, I thought I would send along the following comments

 

  1. The SC1 chart is like a map of the surface of the Earth.  In the case of the Earth,  we are trying to portray the outer surface of a sphere on a flat piece of paper.  In the case of a constellation chart,  we are trying to represent the inside of a sphere (the celestial sphere)  on such a piece of paper.  Strictly speaking,  it is impossible to represent the surface of a sphere on flat piece of paper without huge distortion,  so some kind of scheme or simplification has to be used.  The most common one is to plot the surface of the sphere between 60 degrees south latitude and 60 degrees north.  Thus most maps of the Earth don’t show northern Greenland,  and not all of the sky is on your SC1 chart.  In particular,  the north star is at declination 90 degrees, and doesn’t show up on your map. 
  2. The number 2000 on the chart refers to the “epoch” or time when the coordinates of the stars are valid.  As discussed in the lecture notes,  the rotation axis of the Earth “precesses”, or wobbles around with a period of 26,000 years.  So the Right Ascension and Declination of stars slowly change with time.  The convention in astronomy is to refer to the coordinates as they were in 1950, or (more often nowadays) 2000.  These are the coordinates used in your chart.  I should emphasize that the motion in 50 years isn’t much,  and you have to be a pretty sharp observer to note the difference in the coordinates, but it can be done. 
  3. Here is a comment about how to orient the chart when looking at the sky.  The heavy line in the middle, separating the top half from the bottom half, is the celestial equator,  which is discussed in the lecture notes and the textbook.  Here is Iowa,  the celestial equator is an imaginary line that goes across the sky from due east to due west,  and intersects the meridian at an altitude angle of about 48 degrees.  So if you stand facing south and hold the chart up at about about a 48 degree angle from the southern horizon,  you will be about right.  The right side of the map will appear to your right (west), and the left side of the map will be to your left (east).  The stars to the west have already passed the meridian (transited); the stars to the east will transit later in the night.  Objects with a Declination of about 40 degrees (like Vega) will be nearly overhead. 
  4. The ecliptic is the wavy line across the chart, swerving above the celestial equator and then below.  It is the projection on the celestial sphere of the plane of the Earth’s orbit around the Sun.  It is on this path, or close to it,  that you will always find the Sun, the Moon,  and the major planets.  The dates given along the ecliptic correspond to the position of the Sun on that day.  For example,  today, September 4,  the Sun is in the constellation of Leo. Find its position at the extreme left side of your chart.  Figuring this out is important for one of the homework problems on the first set. 
  5. Let me describe the coordinates on the SC1 chart.  The scale on the bottom, horizontal axis gives the Right Ascension.  The units used are hours, minutes, and seconds.  Note that 0 hours is right in the middle of the chart.  The Right Ascension scale is repeated on the celestial equator in the middle of the chart.  The other coordinate (corresponding to latitude here on Earth) is Declination.  The scale for it is on both the right and left hand margin of the chart,  and it is repeated at Right Ascensions of 18h, 0h, and 6h.  The vernal equinox is a very important point on the sky,  and is defined as the position of the Sun on the first day of Spring.  It has RA=0h and Dec=0d.   It is smack-dab in the middle of the chart. 
  6. Finally,  the important question comes up : “how do I use this chart to tell me what part of the sky will be visible this evening, or tomorrow morning before the Sun rises”.  Let’s start by recalling the definition of the meridian.  The meridian is an imaginary line across the sky,  starting from due north,  passing through the zenith,  and passing due south.  It splits the sky into two halves,  the eastern half and the western half.  When a star or planet is on the meridian,  it is at the highest altitude angle it achieves.  From the meridian, we will see some stars to the east of the meridian,  and others to the west.  If we know where our meridian is relative to the the stars,  we know how the sky will appear to us.  The SC1 chart tells us where the meridian will be at any time of the day or night,  and time of the year.  Look at the dates on the bottom edge of the chart (not on the ecliptic).  If you put a ruler running straight up and down (vertically) on today’s date,  that shows you where the meridian will be at 8PM standard time (9PM Daylight Savings Time).  So, for September 4,  we would put our ruler about a third of the way in from the right edge,  just to the right of the 19h Right Ascension mark.  This shows us that at 9PM CDT tonight,  the bright star Vega will be nearly on the meridian.  The bright star Altair in the constellation of Aquila (RA=19h45m, DEC=10d) will be a little to the east of the meridian, the constellation of Ophiuchus will be to the west (right),  and the bright star Arcturus (RA=14h, DEC=+20d) will be way west of the meridian.  Try it out tonight!