ASTR 101 Orbit of Mars-1 (1) (1)

Clark College Astronomy 101 Laboratory 04 - Orbit of Mars The Orbit of Mars Materials : protractor, compass, centimeter ruler, engineering paper Object : To reconstruct the approximate orbit of Mars by using some of Tycho Brahe's data. During the latter part of the 16 th century, Tycho Brahe collected a very large number of precise observations of the motion of Mars through the sky. He recorded the position of Mars against the background stars to within an arcminute, and the time of the observation to the minute. Below is a list of some of Tycho's data Date Heliocentric Longitude of Earth Geocentric Longitude of Mars 1585 Feb 17 159° 135° 1587 Jan 05 115° 182° 1591 Sep 19 6° 284° 1593 Aug 06 323° 347° 1593 Dec 07 86° 3° 1595 Oct. 25 42° 50° 1587 Mar 28 197° 168° 1589 Feb 12 154° 219° 1585 Mar 10 180° 132° 1587 Jan 26 136° 185° Once every martian year (687 days) Mars returns to the same point in its orbit around the Sun. If we plot the lines of sight from the earth to Mars at this interval we can triangulate a point on the orbit of Mars. 1. Turn the graph paper so that the side without the lines is facing up and the long edge is horizontal. Place a small dot near the center of the page. Label the dot "Sun". 2. Using the ruler, draw a straight line to the right, starting at the Sun and ending roughly two centimeters from the right side of the page. Label this line the “vernal equinox”. This is the direction an observer on the earth would look to see the Sun on March 21. All angles will be measured counter-clockwise from this line. 3. Using a compass or circle guide, draw a 5.0 cm. radius circle centered on the Sun. Label this circle "Earths Orbit” (figure 1) We know from our studies that the earth's orbit is actually an ellipse. You will see the effect that this difference has on our model when you finish constructing the orbit of Mars . 4. This sets the scale of out drawing at 5.0 cm. = 1 A.U . Where 1 A.U. is the average distance between the Earth and Sun (93 million miles or 1.5 x 10 8 km). 5. Note that the data table is divided into pairs of dates. Each pair represents an interval of one martian year. Starting with February 17, 1585, use a protractor to plot the heliocentric longitude of the Earth (159 o ) given in the table as a point on the earth's orbit (figure 2) 6. Next center the protractor on the point you just marked and plot the geocentric position of Mars. (135 degrees) Orbit of Mars 1/7

Clark College Astronomy 101 Laboratory 04 - Orbit of Mars 7. Now repeat for the Jan. 5th 1587 date. Label the positions of the Earth with the date (you can omit the year). The point of intersection is the position that Mars had occupied on these two dates. Draw a dot there to represent Mars. Label this as position “ 1 ” FIGURE 2 FIGURE 1 8. Repeat this procedure for the next four pairs of data, numbering each point in sequence. You should have 5 positions for Mars. 9. Kepler chose the first two sets of data to represent aphelion and perihelion for Mars, respectively. Draw a line from the first position for Mars to the second position for Mars. This line should pass close to the Sun. This line is the major axis of the orbit. 10. Measure to the middle of the major axis Mark the center of the major axis and label it “ midpoint ”. 11. Using the compass draw a circle representing Mars' orbit by placing the point of the compass on the midpoint (make sure something is under the point to protect the desktop) and draw a circle which passes through both perihelion and aphelion points (the first two points you plotted). 12. The circle should come quite close to the other three points you plotted, but will not intersect any of them. 13. Once more, measure the distance from the midpoint of the major axis to either end (aphelion or perihelion) in centimeters. Label this length " semimajor axis " and record its value on your data sheet FIGURE 3 Orbit of Mars 2/7