THE EFFECTS OF TWILIGHT

References to the daylight "half" and the night-time "half" of Earth are really just convenient simplifications.  In actuality, less than half of Earth is in darkness at any time due to the following factors.  First, the Sun is immensely larger than Earth.  So, on purely geometric grounds, its light "wraps" slightly more than halfway around Earth, such that about 180.52° of our globe is "directly" illuminated.  Second, atmospheric refraction displaces objects at the horizon "upward" by an average of about 34' (arc-minutes) or about 0.57°.  This, along with upper atmospheric scattering of light, produces the twilight that illuminates the surface beyond the "half-Earth boundary".

Twilight is typically divided into the three regions shown above.  These translate into periods of time due to Earth's rotation on its axis.  However, because of our planet's spherical shape and the orientation and extent of its shadow, the length of twilight at different locations can be quite different.  The most extreme example is at the poles, where twilight lasts for weeks!

It is Civil Twilight when the Sun is below the horizon by not more than 6°.  When skies are clear, it is still easy to distinguish the entire horizon around you, though the brightest planets and stars are visible.

It is Nautical Twilight when the Sun is between 6° and 12° below the horizon.  Nautical Twilight's name derives from the fact that at sea it still allows navigational readings of star and planet positions relative to the horizon.  But by the end of Nautical Twilight, the horizon becomes too dim for accurate readings.

It is Astronomical Twilight when the Sun is between 12° and 18° below the horizon.  Though the sky appears dark during this time, there is still enough diffuse light in the atmosphere to hinder the observation of Dark Sky objects and meteor showers.

In the diagram above, the brightness of the bands of twilight has of course been exaggerated.  In reality, illumination drops off gradually.  Nonetheless, sizes of the three bands of twilight are shown accurately, and it is easy to see that twilight can significantly reduce the time available for viewing the night sky.  For this reason, many of the views of Earth on the SkyMarvels™ site try to take twilight into account to provide a more accurate depiction of our globe's shadow.  You can see this on our  Earth Surise & Sunset  page and on our  Equinoxes & Solstices through March 2014  page.


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