EQUINOX
This is a period of the year when the sun is above the equator while day and night are of equal length. It usually occurs around 200 March (which is called spring equinox) and 22nd September (which is called autumn equinox).
SOLSTICE
This is another period of the year when the sun reaches its highest or lowest point in the sky at midday. It is also a period when day and night are of unequal length. It usually occurs around 21st June and 22nd December every year. This period can be said to be when the sun is vertically overhead at the tropics of cancer and Capricorn.
TWILIGHT
This is also a period of time usually at the end of the day when the sun goes down the horizon. It is a time of faint light or when the sun sets.
DAWN
This is a time of the day when light first appears i.e. — daybreak. It is a brief period between sunrise and full daylight.