Stargazing Time

Ask yourself this question: "When do the stars come out?"  A first answer to this question might be "when the sun goes down."  I'm not going to say this is wrong...clearly you can see that stars at night.  However, I would say this is a rather inexact statement.  Think about this - how long after sunset does it take for it to get dark outside?  This period of time between sunset (or sunrise) and night is called twilight, and it has nothing to do with sparkly vampires.

As shown in the figure to the left, there are three different categories of twilight:

1) Civil Twilight: This is the brightest of the 3 since it is the closest to sunset or sunrise.  It lasts from when the sun is just below the horizon until it is 6 degrees below.  The name civil twilight comes from the fact that many laws define night to start at the end of this period.  For example, motorists must begin to use their headlights and pilots need to licensed to fly at night.  During this period of time, earthbound objects are usually distinguishable without artificial light while only the brightest stars and planets are visible.

2) Nautical Twilight: This twilight lasts until the sun is 12 degrees below the horizon, so it gets darker than civil twilight.  Nautical twilight got its name from the days when sailors would have to use a sextant for navigation.  It is the time where the sky is dark enough so well-known stars are visible for sighting while it is still light enough to distinguish the horizon for measuring elevation.

3) Astronomical Twilight: This period last until the sun reaches 18 degrees below the horizon, at which point the sky is dark enough for any type of astronomical observation.  Up until this time, most stars can be observed while some galaxies and nebulae cannot.  The sky can appear to be completely dark near the end of astronomical twilight.  Due to the effects of light pollution in populated areas, many observers don't see a sky darker than this.

It's more enlightening to see how twilight works in terms of an actual day, so I got a table of the twilight times in relation to sunrise and sunset for today (in the Denver area).  Of course, these times will change everyday like sunrise and sunset do.  They also depend on the latitude of your city.  If you're interested, click here and then on "more" in the bottom table to see a table for the current date at your location.  See...isn't this twilight so much better?