Sunday, October 09, 2005

A Year of Twilight: A Graphical View

The graphic below will be used to create charts showing when a celestial object is above a certain altitude (i.e. 30 & 50 degrees above either horizon). The thick black line shows when the star is transiting the meridian.

Overlaying the three twilights shows when the star is 'at altitude'.

[Image] Twilight Chart
[Image] γ Cas - Transit & 30°/50° Alt.


• Civil Twilight (outside lines) is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions in the absence of moonlight or other illumination. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities. Complete darkness, however, ends sometime prior to the beginning of morning civil twilight and begins sometime after the end of evening civil twilight.

• Nautical Twilight (middle lines) is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct.

• Astronomical Twilight (inside lines) is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening the Sun does not contribute to sky illumination; for a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible.

Definitions from 'U.S. Naval Observatory: Rise, Set, and Twilight Definitions'.


Tag said...

Hi Ben,

Thanks for the charts, the 30° indication is a nice touch. This is from your Excel handiwork, no doubt.

When I looked at this initially, the twilight template was only available. I was uncertain if it was to be printed off as a transparency and overlayed onto something else. Now I see its application with gamma Cas example.

Do have any particular stars or objects in mind that you'll post?

c'ya soon. peter

Ben C. said...


Thanks for the feedback.

I have a number of objects in mind including NELM areas to produce charts for (i.e. in the N, S, E, W etc.) showing if they will be up for that evening.

What I wanted to do this project all along was to see when the various Search Area Location Charts for hunting nova will be at the right altitude for observing.

I've added a feature that I'll upload tonight that will show when the area gets above 30 degrees and then when it hits 50 degrees which is when it becomes difficult to use the tripod mounted binoculars.

It so clearly shows, at a glance, what time of year the area is best suited for observing at any time in the evening. I think the blog kicked this project into high gear.

And yes, this is all done with Excel.

All the best.