By learning the albedo geography , you can watch for changes on Mars . The polar caps grow and recede . There can be a seasonal “ wave of darkening .” White clouds and hazes come and go . And changing dark features may be obscured by dust storms , which occasionally expand to become literally planet-wide .
22 other colors through . And studying the transmission curves of filters , or simply viewing through filters called the same color may show that their transmissions can be different , even with the same name . A red filter transmits only the color red , but a deep red filter transmits only redder reds , not those close to orange . An orange filter transmits orange and red and a yellow filter transmits yellow , orange , and red . A green filter removes red and blue and blue filters transmit violet and blue ( and , if they have the common red “ leak ,” they sometimes transmit a deep red ). Some filters use thin films of chemicals in layers on the glass to select transmitted and blocked colors ( more correctly described as wavelengths of light ). For example , the common nebular and light pollution reduction filters work this way . More recently , “ thin film ” filters have become available that improve the contrast of planetary surfaces or cloud top belts and zones on Jovian planets . Filters for Mars improve the contrast of Martian geography and clouds in its atmosphere . Colored filters also reduce the effect of dispersion by taking out some of the colors that otherwise smear over each other in an unfiltered view of Mars ’ disk . There are long lists of filters identified for improving the view of Mars for specific geography or meteorological phenomena but , to get started , you can do well with one , two or three filters , depending on your level of interest and bank account ( Figure 3 ). Not among the usual recommendations is a neutral grey filter . These filters may be called just that , or they are sometimes referred to as moon filters . A single polarizing filter can also be used for this purpose . A neutral grey filter reduces all the colors of the light passing through it equally ( hence the “ neutral ,” with grey indicating the filter ’ s “ color ”). When used on Mars , the bright glare of the planet against the dark background sky is reduced , allowing your eye to look for the low contrast geographic differences across the small disk of Mars in its natural colors . A red filter is useful to increase the contrast between the dark geographic markings on Mars and the surrounding lighter areas . While the markings are not green , they do seem to get darker ( as green would ) when seen through the filter . Yellow dust clouds , white clouds and the white polar caps are lost when only red light makes it through the filter . A blue filter , in some ways , has less direct use . But to see the Martian atmosphere ( most of the time ), a medium or deep blue filter emphasizes the thin carbon dioxide air on Mars . High white clouds have a better chance of standing out with a blue filter . Infrequently , events called blue clearings occur and Martian surface features can then be seen through the blue filter .
Training Your Eyes Mars merits multiple viewings over single nights as well as across multiple weeks . Mars presents seasonal changes ( due to the tilt of its rotation axis as well as due to its changing distance from the Sun over it orbital revolution ). Over the course of the night , Mars ’ rotation will be apparent as features slowly move across the disk . But to see the complete globe , several weeks of observing will be involved . That ’ s because Mars ’ rotation rate is only a little longer than Earth ’ s . So that difference leads to an offset ( about 7.5 °) of the central meridian ( longitude ) on Mars if you start observing at the same clock time each night . It takes time for the change to be really noticeable and additional time each night to see more of Mars ’ geography . It is helpful to determine what part of Mars is visible before going out to observe . You can search “ Mars central meridian calculator ” in your browser or use the one linked here . This one allows you to jump ahead ( or backward ) in time and even presents the map of the visible hemisphere to match your telescopic view . ( Depending on your telescope ’ s design and the accessories you use with it , Mars may appear with North up or down and maybe mirror-reversed . Click the appropriate button .) This is very handy as you learn to see Martian geography and then to identify it . Set up your telescope and observing chair and point the telescope to Mars . Start by figuring out the cardinal directions in the field of view . If you are using an AltAz mount ( like a Dobsonian ), let the planet drift through the field of view . The direction of travel is from east ( E ) to west ( W ) on the sky ( opposite the directions on a road map ). Once E-W is established , move the telescope tube perpendicular to E-W . In Earth ’ s northern hemisphere , pushing the telescope upwards moves the south pole of Mars towards the edge of the field first . With an AltAz mount , the cardinal directions will shift with respect to the orientation of your focuser ’ s knobs over a few hours .
Sky ’ s Up