One screen of graphics is drawn per star. This consists of a header, a large plot, and some lines of text explanation. The plot is formatted with date through the year shown horizontally and time through the day shown vertically. The left-hand scale shows UT and the right-hand scale shows local time with midnight at the centre. Figure 1 shows a summary of the symbols used in the plot.
| —X— | start of astronomical night |
| —V— | end of astronomical night |
| —— | star rises |
| —— | star sets |
| —— | star rises above 30 elevation |
| —— | star falls below 30 elevation |
| - - - - | great circle Moon-star distance |
| | 25%–50% illuminated Moon above horizon |
| | 50%–75% illuminated Moon above horizon |
| | 75%–100% illuminated Moon above horizon |
Wavy horizontal solid lines show the start and end of the astronomical night. Symbol X shows the start of the night (i.e., the end of day-to-night astronomical twilight), and symbol V shows the end of the night (i.e., the start of night-to-day astronomical twilight). For extreme northerly and southerly geographical locations these lines merge to become a closed shape as there is no astronomical night during polar summer.
Straight diagonal solid lines marked with symbols show the times of star rising () and setting (). Parallel lines show the times at which the star rises above () and sets below () 30 elevation, a typical minimum elevation for useful observing.
The remaining lines in the plot refer to the Moon. The great-circle distance of the Moon from the star is shown (in hours) by a dashed sinewave in the top half of the plot. Read the distance off the right-hand scale and multiply by 15 to get the distance in degrees.
The rising and setting of the Moon is shown by vertical solid lines of variable width. A line is plotted for every other day, it starts at the time of moonrise and ends at the time of moonset. The width of the Moon lines are related to the phase of the Moon for that date. The thickest lines are plotted when the Moon is 75%-100% illuminated, thinner lines are plotted for 50%-75% and 25%-50% illumination. No line is plotted when the fractional illumination is less than 25%.
The region of visibility of the star typically falls halfway down the plot (between the wavy lines), and is bounded on left and right by diagonal lines with empty and filled circles superimposed. Depending on the nature of the observation planned, you may then need to look for dates and/or times clear of the vertical Moon lines.
If you reply ’yes’ to the TEXTF parameter, observe will create a text file that contains the star rise and set times, the times at which the star rises above or moves below 30 above the horizon, the times when astronomical twilight begins and end, the distance star/Moon separation and the moon phase for every day of the year chosen.