The period between the Sun being 12 and 18 degrees below the horizon.
Comes after evening and before morning nautical twilight. At the end of this all astronomical objects, including deep sky objects such as galaxies and nebulae, can be properly observed.
See also Civil Twilight | Nautical Twilight
The period between the Sun being 6 degrees below the horizon and sunrise or sunset.
In the evening it starts at sunset and ends when the centre of the Sun has sunk 6 degrees below the horizon. In the morning it starts when the centre of the Sun is 6 degrees below the horizon and ends at sunrise. During civil twilight the horizon is clearly visible and objects can be seen without extra illumination.
See also Astronomical Twilight | Nautical Twilight
Normal (eastwards) motion of a celestial body
As the planets orbit the Sun they move eastwards against the background of stars. However the Earth also in motion and this changes our viewing point of any planet. Imagine Jupiter coming to a dead stop; it would still appear to drift back and forth against the stars over the course of a year because of our own planets motion. Add these two motions together and the superior planets (Mars outwards) trace loops in the sky rather than smooth orbital paths. Before opposition these planets appears to stop, then move retrograde (backwards) to the west. After opposition they again stop and continue with their direct eastwards motion.
The period between the Sun being 6 and 12 degrees below the horizon.
Come after evening and before morning civil twilight. At sea, navigation using the horizon as a reference is not possible because it is no longer visible. By the end of nautical twilight the sky is dark enough for many astronomical purposes.
See also Astronomical Twilight | Civil Twilight
'Night shining' clouds.
These are wispy clouds visible from high latitudes above the northern horizon for several weeks either side of the summer solstice. White or maybe electric blue in colour, they form in the highest regions of the atmosphere up to 100 km high, on the edge of space. They are made of water vapour but how they form is still controversial. Too tenuous to be seen in the daytime, they are visible in the late twilight light by the Sun below the horizon.
In astronomy, an occultation happens when one object passes in front of and obscures another. The Moon regularly occults stars as it orbits the earth, less frequently it occults planets. Very rarely a planet will occult another planet in the solar system.
Because the Moon is relatively close to the Earth a particular occultation may not be seen from all places, the star or planet may instead appear close to the Moon. Some places may witness a 'grazing occultation' when the star or planet skims the edge of the Moon, going in and out of view behind the lunar mountains.
Backwards (westwards) motion of a celestial body
As the planets orbit the Sun they move eastwards against the background of stars. However the Earth also in motion and this changes our viewing point of any planet. Imagine Jupiter coming to a dead stop; it would still appear to drift back and forth against the stars over the course of a year because of our own planets motion. Add these two motions together and the superior planets (Mars outwards) trace loops in the sky rather than smooth orbital paths. Before opposition these planets appears to stop, then move retrograde (backwards) to the west. After opposition they again stop and continue with their direct eastwards motion.
See also Winter Solstice | Summer Solstice
The Earth's axis of rotation is tilted by nearly 23 degrees from it's orbital plane round the Sun. The globe acts as a huge gyroscope and this tilt is maintained in the same direction throughout the year long orbit. Currently the north pole is aligned roughly with Polaris, the North Star, though this alignment does slowly change in a phenomenon known as precession.
The consequence of the tilt is that for half the orbit the northern hemisphere is tilted towards the Sun and the other half of the orbit it's tilted away.
Seasons are caused by the 23 degree tilt of the Earth
In northern summers the Sun is north of the plane of the Earth's equator and shines high in the sky at noon. It rises north of east and sets north of west, staying above the horizon for more than 12 hours. In winter the opposite is true; the Sun is south of the plane of the Earth's equator, shining low in the sky at noon. It rises south of east and sets south of west, staying above the horizon for less than 12 hours.
The Sun is highest at the summer solstice and lowest at the winter solstice. At the spring and autumn equinoxes the Sun is overhead at the equator and there is an equal 12 hours of day and night over the globe.
The summer and winter differences are more pronounced nearer the poles. Past the Arctic and Antarctic circles the Sun never rises above the horizon for a period either side of the winter solstice and in the summer it never sets for an equivalent period round the summer solstice. The closer to the pole, the longer these periods are. At the poles there is no 24 hour period that can be called day and night; the Sun is below the horizon for six months, rises in the spring and stays above the horizon for the remaining six months, to set in again autumn.
See also Winter Solstice | Seasons
At the northern hemisphere summer solstice the Sun reaches it's furthest point north in the sky and the north pole is angled in line with the Sun and towards it. In Britain and northern Europe we have long hours of daylight and short night with the Sun high above the horizon at noon.
At the north pole at any time of year the stars (including those of the northern zodiac) turn round the north star but never rise or set. The summer Sun does not set either, and does not disappear until autumn when it moves into the southern sky.
A: At the summer solstice on the Arctic circle (23.4 degrees north) the Sun just grazes the horizon at midnight, but never fully sets and it is daylight for 24 hours.
B: South of the Arctic circle the Sun sets and the further south you go, the longer the time the Sun is below the horizon at night. In the UK the Sun is never far below the northern horizon and we experience twilight extending through the night.
The noon time Sun gets higher in the sky the further south you go. The height of the midday Sun on the summer solstice depends on your location. In Scotland it is around 56 degrees above the horizon, in the southern UK it can reach 62 degrees.
At the same time it is the winter solstice in the southern hemisphere and the situation is opposite to that in the north.
See also Summer Solstice | Seasons
At the northern hemisphere winter solstice the Sun reaches it's furthest point south in the sky and the north pole is angled away from the Sun. In Britain and northern Europe we have long hours of night and short days with the Sun low above the horizon at noon.
At the north pole at any time of year the stars (including those of the northern zodiac) turn round the north star but never rise or set. The winter Sun does not rise either, and does not appear until spring when it moves into the northern sky.
A: At the winter solstice on the Arctic circle (23.4 degrees north) the Sun just grazes the horizon at noon, but never fully rises and most of the day is in darkness. The noon time Sun gets higher in the sky the further south you go.
B: The height of the midday Sun on the winter solstice depends on your location. In Scotland it is around 10 degrees above the horizon, in the southern UK it can reach 16 degrees.
At the same time it is the summer solstice in the southern hemisphere and the situation is opposite to that in the north.