Opposition of MarsJune 14, 2001
Image created from http://space.jpl.nasa.gov/ |
|
Mars, the red planet, holds a great fascination for humans. Associated with the God of War, this fascinating world has been the subject of endless speculation on whether life once inhabited it, and is currently the target of a bevy of spacecraft making for its frigid and arid surface.
|
Diameter: 6794 km Moons: 2, Phobos and Demios (Fear and Terror) Year: 687 days Day: 1.026 Earth days Mass: 0.107 Earth Mass (0.64 x1024 Kg) |
|
Alert! Mars is currently in the grip of a huge planet-wide dust storm, obscuring all but the most obvious features from our telescopes.
This year is the best opposition of Mars since 1988, and this is an excellent opportunity for people to observe this fascinating world. What is an opposition? Opposition refers to when a planet is opposite the Sun in the sky, this can only happen to outer planets, as Earth must pass between the Sun and the planet. The Earth passes Mars in its orbit every 26 months, and at this time we get a good view of the Red Planet.
When Mars is also making its closest approach to the Sun, our view is very good indeed. While Mars is on average 228 million kim from the sun, due to Mars's eliptical orbit this varies by 42 million kilometers. If Mars is at its furthest from the Sun at opposition, Mars is also around 99 million km from Earth, while if Mars is at its closest to the Sun during opposition, this value narrows to only 57 million km. Favorable oppositions occur only once in every 15 to 17 years. This years is the best for thirteen years, with Mars a mere 67.3 million kilometers from us, but will be surpassed by the opposition of 2003 when Earth and Mars are a mere 55.8 million km apart.
Oppositions in the early months of the year, when Mars is furthest from the Sun, are allways poor. The best oppostions occcur around August. This is very good for Southern observers, as Mars is high in the sky, and the winter sky is usually still and transparent, ideal conditions for watching Mars. During this years opposition, the visible disk of Mars increases in diameter, from 5 arc seconds, to 20.8 arc seconds (an arc second is approximately 1/3600th of your fingerwidth). While this sounds astoundingly small, 20.8 arc seconds will give quite a substantial disk in most amateur telescopes, even the small ones from Tasco. The actual opposition is on June 14, but Mars does not obtain maximum diameter until June 22.
|
What you can expect to seeNaked eye. At 10 pm AEST Mars is almost directly east, and 9 handspans above the eastern horizon at the begining of June (when your hand is held with your arm outstreched, your hand covers 5 degrees of sky, see diagram at right, people in Brisbane and Sydney should add an extra handspan, from Darwin Mars is to the southeast). By mid June it is 12 handspans above the eastern horizon and by the end of June it is almost directly overhead. Mars starts June at magnitude -2.0, and is the brightest object in the evening sky. During June Mars will visibly brighten to magnitude -2.4 by the 14th, then slowly fades during the rest of June and over July. Between August and October Mars then fades rather sharply. Mars is located in Ophiuchus, near the hooked tail of Scorpio and the "teapot" of Saggitarius, where there are a number of naked eye clusters and a great swathe of the Milky Way. Mars looks particularly beautiful in this setting. Plotting the position of Mars every few nights (use a torch covered in red cellophane to stop your night vision being destroyed while you draw its position) will be interesting. During June, Mars moves towards Scorpio, then in July it abruptly reverses direction and moves towards Saggitarius. This is called retrograde motion, and occurs due to Earth overtaking Mars in its orbit. A map showing this motion is here. On the 21st and 22nd of June, Mars will be a quarter of a fingerwidth from the dim star 36 Ophiuchi, and on the 15th of August Mars will pass within half a fingerwidth of 36 Ophiuchi again. On 15 September rapidly fading Mars passes within two fingerwidths of the Lagoon and Triffid nebula.
|
 View of the eastern horizon at approximately 10.00 pm AEST, 15 June, as seen from Melbourne. The view will be similar from most Australian locations at 10.00 pm local time. |
|
Binoculars Mars is a clearly visible disk (although markings are unlikely to be seen with standard binoculars). As Mars is within the Milky Way, Mars looks very attractive in binoculars with many small clusters being visible in a binocular field. The retrograde motion map also shows many of these clusters. On the second of July Mars is half a fingerwidth from the magnitude 7.0 globular cluster M19, and should look attractive under dark skies. During August and September, Mars is within a binocular field of several attractive clusters and nebula. Telescopes Mars shows clearly visible markings in a 50 mm refractor telescope, and significant detail can be seen in a 4" reflector, while 6" and 8" instruments will give better detail still. No current Earth-bound telescope can reveal the huge volcano, Mons Olympus, or the huge valley of Vale Marensus, which are seen in many of the spacecraft images. However, significant features such as Syrtis major (featured in the Masthead graphic) will be visible in even a small telescope. The Martian day is about 40 minutes longer tha the Earth day, so if you observe at the same time each night, you can see the surface features rotating into and out of view. Dust storms can also occur, sometimes lasting days. Wind removal and deposition of the reddish, iron rich dust can also reveal or obscure features, so Mars's appearance can be somewhat different between each opposition. Seasonal winds alternatly covering and uncovering darker features with lighter dust were once interpreted as seasonal plant growth. Studing the Martian storms and the changing surface features is a valuable amateur activity. In the May 2001 issue of SKY & TELESCOPE (pages 115 to 123), Thomas Dobbins and William Sheehan discussed rare historical observations of bright, star-like flares from certain regions on the planet Mars. They suggested that the flares might be caused by specular reflections of sunlight off water-ice crystals in surface frosts or atmospheric clouds, specifically at times when the sub-Sun and sub-Earth points were nearly coincident and near the planet's central meridian (the imaginary line running down the center of the visible disk from pole to pole). These flashes have been seen by observers in the US in Edom Promontorium, near the Martian equator. 3-5 second flashes were seen over a 90 minute period. Australian observers might like to try looking for them. The flashes began at 6:40 UT on June 7; since Mars' rotation period is 24h 37.4m, then on June 9 UT they should begin at about 7:15 UT, on June 10 at 7:52, on June 11 at 8:30 UT, etc. It's not known how long they will be visible; on June 7, they were seen for 90 minutes before clouds moved in and prevented further observation. The changing subsolar latitude on Mars will modify the times above and eventually cause the flashes to no longer be visible. So June is an excellent time to dust off that old telescope lying around in the garage, or to beg a view from a friend or neighbour with a telescope. Better yet, many astronomical clubs hold open nights, and this is an excellent opportunity to see this fascinating world in a decent telescope. Also, some of the local planetariums may be showing off Mars if they have telescopes (See the Links section for addresses). For recording the apearance of Mars, all you need is a sheet of paper on a sturdy back ground, a pencil (or coloured pencils if you want to try recording the colors you see), a small torch coverd in red cellophane and a watch. Make sure you and your telescope are located in a relatively dark place, and have modest circles predrawn on your paper (I use a 20 cent piece). Have your telescope out for a while beforehand so that it is at ambient temprature, to prevent air currents in the telescope from ruining the image. Record the date and time, and the weather (if it is windy, how much cloud, how much moonlight, what is the dimmest star you can see, etc.). Make sure you are wearing warm clothing, then make yourself comfortable at the eyepiece, preferably with a chair that allows you to sit and view comfortably, and, well, start drawing. It may take a few trys before you get the hang of recording what you see by red light, but you will feel a warm glow of accomplishment when you can. The Ephemeris below gives the time Mars rises, its magnitude, and its altitude at midnight (5 degrees is equivalent to the distance covered by an outsreached hand), and its aparent diameter.
| |
