# What are the relative positions of Earth and Mars?

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#### keermalec

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Hi,<br /><br />i seem to find contradictory data on the argument of perihelion of Earth and Mars on the Internet.<br /><br />Can anyone point me to a valid source of data for:<br /><br />1. Argument of perihelion for Earth and Mars<br />2. Angular position of Earth and Mars at a given date<br /><br />Many thanks! <div class="Discussion_UserSignature"> <p><em>“An error does not become a mistake until you refuse to correct it.” John F. Kennedy</em></p> </div>

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#### bdewoody

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Our relative position varies as the two planets orbit the sun a different rates. <div class="Discussion_UserSignature"> <em><font size="2">Bob DeWoody</font></em> </div>

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#### h2ouniverse

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Sorry Thalion but they give the longitude of perihelion, not the argument of perihelion.<br /><br />Longitude of periapsis = Argument of periapsis + Longitude of Ascending Node.<br /><br />I think this is probably the reason for the confusion.

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#### h2ouniverse

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in reply to:<br />-------<br />"Angular position of Earth and Mars at a given date "<br />------<br /><br />If you are courageous and do not want to use JPL 's Horizon S/W http://ssd.jpl.nasa.gov/horizons.cgi you can compute both orbits from the Keplerian parameters and make your own ephemeris. Use equation MT = E - e sinE and iterate E=>T=>E. That is doable with Excel with a reasonable accuracy after just two or three iterations.<br /><br />

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#### 3488

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Dunno if the below is of any use??? <font color="yellow">(Probably about as much as an icecream barbecue)</font> <img src="/images/icons/crazy.gif" /><br /><br />View of Sun from Earth @ Perihelion. Friday 4th January 2008.<br /><br />View of Sun from Mars @ Perihelion. Saturday 18th April 2009.<br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

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#### keermalec

##### Guest
Thanks Thalion and H20. If I got it right then, <br /><br />argument of perihelion = longitude of perihelion - longitude of the ascending node. Therefore:<br /><br />Earth argument of perihelion = 102.94719 - -11.26064 = 114.20783Â°.<br /><br />Mars argument of perihelion = 336.04084 - 49.57854 = 286.4623Â°<br /><br />Wikipedia was actually right!!!<br /><br />Now concerning their angular positions at a given date, using JPL Horizons I get this cryptic result when positioning the observer at the center of the sun and looking at Mars on December 28, 2007:<br /><br />***********************************************************************************************************<br /> Date__(UT)__HR:MN R.A._(ICRF/J2000.0)_DEC APmag S-brt delta deldot S-O-T S-T-O<br />***********************************************************************************************************<br />\$\$SOE<br /> 2007-Dec-28 00:00 06 17 48.53 +24 40 23.6 0.47 4.07 1.57948401521819 1.9985202 0.0000 0.0044<br /><br />And when looking at Earth:<br /><br />2007-Dec-28 00:00 06 24 59.70 +23 18 50.0 n.a. n.a. .983369465924225 -0.0540944 0.0000 0.0058<br /><br /><br />Can someone explain how I can extract the longitude of observed planet from the above data?<br /><br />Many thanx again!<br /> <div class="Discussion_UserSignature"> <p><em>“An error does not become a mistake until you refuse to correct it.” John F. Kennedy</em></p> </div>

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#### keermalec

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OK I think I got it!<br /><br />I changed angular units from HH:MM:SS to degrees and came up with this for Mars as seen from the center of the sun:<br /><br />*******************************************************************************************************<br /> Date__(UT)__HR:MN R.A._(J2000.0)_DEC. APmag S-brt delta deldot S-O-T S-T-O<br />*******************************************************************************************************<br />\$\$SOE<br /> 2007-Dec-28 00:00 94.45223 24.67322 0.47 4.07 1.57948401521819 1.9985202 0.0000 0.0044<br /> 2008-Jan-27 00:00 110.07330 23.75331 0.55 4.11 1.61146267827309 1.6758702 0.0000 0.0043<br /> 2008-Feb-26 00:00 124.77199 21.41366 0.62 4.14 1.63705644698968 1.2663692 0.0000 0.0043<br /> 2008-Mar-27 00:00 138.52872 17.95196 0.67 4.17 1.65500384992017 0.7975487 0.0000 0.0042<br /> 2008-Apr-26 00:00 151.47939 13.64403 0.69 4.18 1.66449288265116 0.2938758 0.0000 0.0042<br /> 2008-May-26 00:00 163.86000 8.72414 0.69 4.18 1.66511544033784 -0.2222875 0.0000 0.0042<br /> 2008-Jun-25 00:00 175.96289 3.39014 0.67 4.17 1.65684263708815 -0.7293095 0.0000 0.0042<br /> 2008-Jul-25 00:00 188.11548 -2.17658 0.63 4.15 1.64002350202430 -1.2049593 0.0000 0.0043<br /> 2008-Aug-24 00:00 200.67516 -7.78232 0.56 4.11 1.61540731160394 -1.6250568 0.0000 0.0043<br /> 2008-Sep-23 00:00 214.02643 -13.18505 0.48 4.07 1.58418796761388 -1.9623313 0.0000 0.0044<br /> 2008-Oct-23 00:00 228.55892 -18.05378 0.38 4.02 1.54806024326395 -2.1863296 0.0000 0.0045<br /> 2008-Nov-22 00:00 244.59333 -21.93568 0.27 3.96 1.50926502893814 -2.2648934 0.0000 0.0046<br /> 2008-Dec-22 00:00 262.22758 -24.26604 0.15 3.91 1.47058882770799 -2.1681655 0.0000 0.0048<br /> 2009-Jan-21 00:00 281.15273 -24.47664 0.05 3.86 1.43526124942951 -1.8764976 0.0000 0.0049<br></br> <div class="Discussion_UserSignature"> <p><em>“An error does not become a mistake until you refuse to correct it.” John F. Kennedy</em></p> </div>

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#### thebigcat

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And to think Kepler figured all that out without a computer. <img src="/images/icons/laugh.gif" /> <div class="Discussion_UserSignature"> </div>

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