GraphDark Hint - Altitude Masks

  For most objects in GraphDark, apart from 'All planets', the time it is above the horizon is represented by a single coloured bar. Altitude masks overlay this bar with different colours to represent the altitude ranges - different masks can be created with varying altitude ranges and colours. Using masks can give a better idea of how well placed an object might be for observing. Just because an object is above the horizon for several hours doesn't mean it rises very high.

Taking Saturn as an example. Currently, in 2004, it is well placed for northern hemisphere observers - but in 10 years time things will be very different.

First choose a northern site, say London, then choose Planet, Saturn. To set an altitude mask click on the M button (between Planet and Saturn). From the form presented choose the mask 'Ten degrees'. Set an end date in 2015 and draw the graph.

    For the time when Saturn is above the horizon the graph is now colour coded according to its altitude.

By comparing it to the colour coded bar along the top of the timetable you will see that currently Saturn rises to over 60 degrees.

To toggle the mask on or off simply click the 'Mask' button at the top edge of the timetable.

    Now move the slider bar, at the bottom of the timetable, to the extreme right to see the situation in 10 years time.

Saturn doesn't even rise above 30 degrees then.
For this example the narrowest possible day width was set to make the changes from year to year more obvious. Go to Edit, Preferences, Timetable display, and put 1 in the 'Pixels' box. Using 'Vertical day lines', also on the Timetable display tab, and maximising the timetable squeezes in more days onto a single screen.

The masks need not represent all altitudes from 0 to 90 degrees. For example the 'Rainbow' mask has just two ranges for altitudes where the Sun is low enough to show the primary and secondary arcs above the horizon. The special 'Haze' mask only has a single colour from the horizon up to the selected altitude - equivalent to Haze in version 1.

The default mask can be changed in Preferences, Startup, What defaults.

   Richard Fleet 2004

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