Although most people tend to think of a day as consisting of just two parts - day and night - there are actually a few other parts to the day.
All of us have, at some point or other, read of, heard about, and even used, the term "twilight". But did you know there are, in fact, three distinct types of twilight?
In general terms, for non-astronomers, twilight is when there is still light outdoors, even though the sun is below the horizon; that time of day, either morning or evening, when the sun's light is scattered into the Earth's upper atmosphere, illuminating its lower portion. In the pre-sunrise period, this is often referred to as "dawn", while in the post-sunset period, it is referred to as "dusk", thought these two terms refer more to a transition point than an actual interval of time.
Astronomers, however, break twilight into three distinct stages - civil twilight, nautical twilight, and astronomical twilight.
Civil twilight, the brightest of the three stages, is when the sun is less than six degrees below the horizon. In the morning, it begins when the geometric center of the rising sun reaches the six-degree point below the horizon (referred to as civil dawn) and ends at sunrise (when the sun's upper limb appears over the horizon). During the evening, it begins at sunset (when the upper limb of the sun disappears below the horizon) and ends when the geometric center of the setting sun reaches the six-degree point below the horizon (referred to as civil dusk). During civil twilight, only the brightest celestial objects are visible.
It should be noted, however, that actual sunrise and sunset times are calculated by taking into consideration the effects of refraction (the bending of light). At these times of the day, the sun's rays enter the Earth's atmosphere at a very shallow angle (relative to an observer looking towards the horizon), so the sun can be seen a few minutes before sunrise actually occurs in the morning, and a few minutes after sunset actually occurs in the evening.
Civil twilight is preceded by nautical twilight. A reference to when ancient mariners navigated by the stars, nautical twilight is the period when the geometric center of the sun is between six- and 12-degrees below the horizon. Nautical dawn is when the geometric center of the sun reaches the 12-degree point below the horizon in the morning, with nautical dusk occurring when it is at the 12-degree point below the horizon in the evening.
Astronomical twilight, of most importance to night sky observers, is when the sun is between 12- and 18-degrees below the horizon. Astronomical dawn (the end of nighttime) occurs when the geometric center of the sun is at the 18-degree point below the horizon in the morning, and astronomical dusk (the beginning of nighttime) is when the geometric center of the sun is at the 18-degree point below the horizon in the evening. During astronomical twilight, the sky is completely dark, and naked-eye celestial objects are visible.
Actual twilight times vary depending on the time of year, and the observer's latitude; areas closer to and at the equator have shorter twilight periods than those closer to the poles. It is possible to go online and find the actual twilight times (for all three stages) for your particular location.
This week's sky
Both Mercury and Venus are too close to the sun to be seen at present. Jupiter, Saturn, and Mars continue as pre-dawn sky objects. Jupiter (magnitude -2.6) is visible shortly after midnight, about seven degrees above the southeast horizon, reaching an altitude of 22 degrees above the southern horizon before fading from view around 5 a.m.
Saturn (magnitude +0.4), to the east (left) of Jupiter, sits 10 degrees above the southeast horizon by about 1:00 a.m., and is highest (23 degrees) in the southern sky by 4:20 a.m., before fading from view around 4:40 a.m.
Mars (magnitude +0.3), rising further east (left) of Jupiter and Saturn, is visible shortly before 2 a.m., reaching 25 degrees above the southeast horizon when it is lost from sight around 4:45 a.m.
Watch the waning moon slide by these three planets during the coming week.
Until next week, clear skies.
Events
June 13 - Last quarter moon
June 14 - Moon at apogee (furthest from Earth)
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers, and anyone who would like to do so is encouraged to email him at [email protected].
