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Journal

No 54 - November 2007

Observing Mars in 2007/08

On the occasion of the historically close Mars opposition in 2003 the Astronomical Society of Edinburgh published two observing guides for the public. These were written mainly by myself and Ken Thomas, and the shorter one was published in this Journal [1]. The longer of the two guides was re-issued for the 2005 opposition. This article is an update for the forthcoming opposition.

The opposition

Mars passes through opposition at Christmas 2007. During the months leading up to opposition the distance between Earth and Mars reduces drastically, and increases afterward. This makes the weeks around opposition the best time to observe the Red Planet.

orbits of Earth and Mars
Fig. 1: Movement of Earth and Mars in their orbits around the Sun. The dots are one week apart, the tracks start on 30 September and end on 16 March. Opposition is at the centre of the tracks, when Sun, Earth and Mars line up. Earth and Mars are closest about six days before opposition. (This figure has been produced with Xephem [2].)

Since the planetary orbits are not circles but ellipses, Mars is closest not exactly at opposition. Also, not every Mars opposition is equally good regarding how close it gets to Earth. In August 2003 it was particularly close at 56 million km. By comparison, during the opposition in 1997 Mars kept the rather longer distance of 99 million km. In 2007 the closest approach is on the morning of 19 December 2007 at 88 million km. The opposition itself is on the evening of 24 December 2007.

The closer Mars and Earth approach each other, the larger Mars appears in our telescopes and the more detail we can see on its surface. Northern observers are unfortunate in that close encounters happen at rather southerly declination. The 2007/08 opposition gives us optimal height above the horizon, but the largest apparent diameter of Mars this time is only 16".

apparent size of Mars
Fig. 2: Size and phase comparison at intervals of four weeks. The dates left to right are 30 September, 28 October, 25 November, 23 December, 20 January, 17 February and 16 March. (The individual views of Mars have been produced with Xephem [2].)

Another consequence of the reduced distance and fuller phase near opposition is an increase in brightness of the planet. Mars is roughly as bright or brighter than the brightest stars. It brightens from -0.1 mag in late September to -1.6 mag at opposition. It then fades to +0.5 mag by mid March. Prior to opposition Mars is closer, appears larger and brighter than after opposition.

Ephemerides

Tab. 1 includes the rise, set and transit times at weekly intervals. During opposition the planet changes from the morning sky to the evening sky. After opposition it rises earlier and at a given time in the evening is at a higher elevation above the horizon.

Mars rises in the East, goes through its highest point in the South and sets in the West. It is a bright, red, object and appears star-like to the naked eye. Don't confuse it with Aldebaran, which is a bright red star in the constellation Taurus. (The author's first recorded observation of "Mars" turned out to be one of Antares, another bright red star whose name translates to "Antimars".) To find Mars by constellations, look between Gemini and Taurus. Aldebaran and the open star clusters of the Hyades and Pleiades are some distance west (right) of Mars, Castor and Pollux are the two main stars of Gemini and are some distance east (left) of Mars. Fig. 3 is a star chart of the area with Mars' path.

star chart with Mars
Fig. 3: Movement of Mars from late September to mid March in the constellations of Gemini and Taurus. The dots are one week apart, the track starts on 30 September and ends on 16 March. The grid lines are at intervals of 5°, the coordinates for J2000. (This figure has been produced with Xephem [2].)

The movement is not straightforward. Mars starts out on the right and moves to the left. In mid November it turns back (becomes retrograde) and only in early February returns to forward (direct, right to left) motion. Opposition occurs near the centre of the retrograde motion. This opposition loop is the result of the two planets running on their circular race track around the Sun. The Earth has the inside lane and the higher speed. Usually it sees Mars apparently moving in the same direction as Earth itself moves, right to left against the star background. But during the overtaking manoeuvre around opposition, Earth sees Mars moving backward against the distant landscape that are the constellations of the fixed stars.

Images with simple cameras

A simple camera can be used to record the position of the planet within the constellations of Taurus and Gemini. You will need a tripod and must be able to expose for a number of seconds up to a minute. Use the ordinary lens for a reasonably wide field. Stars begin to become trails at about 30 seconds exposure time. Use an aperture on stop smaller than maximum as a good compromise between speed and image quality.

dateSunMars mDiPACM
 risesetsetrise transith "° °°
2007-09-3006:1317:5214:2520:5505:4157-0.110+4-26286
2007-10-0706:2717:3414:1220:3905:2658-0.210+5-25220
2007-10-1406:4117:1613:5720:2205:1058-0.311+6-23153
2007-10-2106:5516:5813:4020:0304:5358-0.411+6-22 87
2007-10-2807:1016:4213:2219:4204:3358-0.512+7-21 22
2007-11-0407:2516:2713:0219:1804:1158-0.712+7-20317
2007-11-1107:4016:1312:4018:5103:4758-0.813+7-20252
2007-11-1807:5416:0112:1618:2003:2059-1.014+7-20188
2007-11-2508:0815:5111:4917:4602:5059-1.215+7-20125
2007-12-0208:2015:4311:2117:0802:1759-1.315+6-21 62
2007-12-0908:3115:3910:5016:2601:4160-1.416+5-22 0
2007-12-1608:3815:3810:1615:4301:0260-1.616+3-23299
2007-12-2308:4315:4009:4015:0000:2361-1.616+2-25237
2007-12-3008:4415:4609:0314:1823:3861-1.516 0-26176
2008-01-0608:4215:5508:2513:3923:0061-1.415-1-27114
2008-01-1308:3716:0607:4913:0322:2461-1.214-2-28 52
2008-01-2008:2816:2007:1412:3121:5061-1.013-2-29350
2008-01-2708:1816:3406:4312:0221:2061-0.713-3-29286
2008-02-0308:0516:4906:1411:3620:5361-0.512-2-29223
2008-02-1007:5017:0405:4811:1220:2861-0.311-2-29158
2008-02-1707:3517:2005:2410:5120:0660-0.110-1-29 93
2008-02-2407:1817:3505:0210:3119:4560 0.010-1-28 28
2008-03-0207:0117:5004:4210:1419:2760 0.2 9 0-27323
2008-03-0906:4318:0504:2309:5819:0960 0.4 8+2-26257
2008-03-1606:2518:1904:0509:4418:5360 0.5 8+3-24191
Tab. 1: Full data for the 2007/08 opposition. All times are UT, add one
hour to convert to BST. All data are for Edinburgh. h is the altitude
above the horizon at transit time. m: brightness, D: apparent diameter,
i: inclination of the axis toward the observer, PA: position angle of the axis
(rotation from celestial north), CM: Martian longitude of the centre of the
disc. The rotation of Mars is such that CM increases by 14.6205° per hour.
The tabled values are for 0 hours UT on the morning of the date listed.
(The data have been calculated with Sputnik 2.1.3 [3].)

Drawings at the telescope

In telescopes the planet is visible as a small disc, hopefully with a few dark or bright surface features. Small telescopes are suitable, although the BAA Mars Section [4] recommends at least a 150 mm refractor or 200 mm reflector to make drawings of surface features. Use high magnification with those apertures, 200x to 300x.

Keep observing. As you get used to the view you begin to see more. It may take a while until you start to see surface detail. Observe repeatedly during the night. Mars rotates once in 24 hours and 40 minutes, surface detail will move perceptibly over an hour or so. Keep observing every night. From one night to the next we see almost the same side of Mars, but over a few weeks and with several hours visibility per night all sides will come into view.

Neither pole is tilted towards Earth during this opposition, the polar ice caps will therefore not be easily visible, if at all. But do look out for dark areas on the surface in general. These will show the rotation of the planet.

If you have them, use coloured filters. These can greatly improve the amount of detail you can see. The most commonly used filters are orange/red to enhance the dark surface markings. Green filters help to accentuate the dark band close to the polar cap. Blue/violet filters can sometimes be useful, depending on the state of the Martian atmosphere.

In order that you can make drawings at the telescope, prepare sheets with circles of a standard size, the BAA use 50 mm diameter [5]. Make a note of the time of the drawings, which way celestial North and East are, and the telescope and magnification used. Perhaps also a short comment on the weather or image quality.

Afterward compare your drawings with the maps available on the BAA Mars Section web site [6]. To make the comparison you should know the orientation of the planet. Tab. 1 includes the inclination and position angle of Mars' rotation axis, as well as the central Martian longitude for certain times. Remember that telescopes rotate the image by 180° and that a zenith prism changes this to an upright mirrored image.

Webcam images of the telescopic view

Traditional photography through the telescope can be done, but is difficult. The image of the planet on the film is small and the exposure times are awkward. You need a long focal length, about 10000 mm. To do this use eyepiece projection with a high magnification eyepiece. Even then Mars is only 1 mm on film. A fine grain film may resolve 0.02 mm, so this is not as bad as it sounds. But you will again need extreme magnification when making prints or projecting slides.

If you have a webcam and laptop, it can be quite simple to make a good recording of what you see. Bright planets deliver sufficient light for webcams to work well even with their short frame exposure times of 1/50 second or so. Take little movies of less than a minute, repeat every 20 or 30 minutes to detect rotation. The recording software is not a critical issue, but you should obtain software that enables you to combine a number of movie frames into a single higher quality image. This has to take account of the movement of the image from frame to frame. You should also obtain software to perform an unsharp mask on the combined image. This reduces the limb darkening and increases the contrast of surface features.

Again, you need a long focal length, 3000 to 4000 mm. A 300 mm f/10 Schmidt-Cassegrain telescope can possibly be used in prime focus, but for most telescopes you need to use a 2x Barlow lens, a 2x photo adapter, or eyepiece projection.

References

  1. Astronomical Society of Edinburgh, 2003, Mars Opposition - August 2003, ASE Journal, 46, 8
  2. Elwood Charles Downey, 2000, Xephem 3.4,
    http://www.ClearSkyInstitute.com/xephem
  3. Horst Meyerdierks, 2004, Sputnik 2.1.1,
    http://www.chiandh.me.uk/soft
  4. R.J. McKim, 2002, Mars Section programme, British Astronomical Association,
    http://www.britastro.org/mars/progrm.htm
  5. BAA Mars Section, 1992, Report form, British Astronomical Association,
    http://www.britastro.org/mars/repform1.htm
  6. BAA Mars Section, Maps of Mars, British Astronomical Association,
    http://www.britastro.org/mars/maps.htm

Horst Meyerdierks


Contents

Cover page

ASE questionnaire

SAG magazine and online forum

Book review: Aurora

Observing group returns

Observing Mars in 2007/08

Recent observations

A new dark site for ASE

Forthcoming events

About the ASE Journal


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