A vocabulary of landscape — a landscape of words
1367 entries
sidereal
Sidereal means measured against the stars rather than against the sun. A sidereal day is the time the earth takes to turn once relative to the distant stars, and it is about four minutes shorter than a solar day, because the earth has also moved along its orbit and has to turn slightly further to bring the sun back to the same place. Over a year, the four minutes add up to exactly one extra rotation.
Etymology
The word is Latin, built on the word for a star or a constellation, which also gives consider and desire by routes that ran through the language of augury and star-reading. To consider was, at root, to examine the stars closely.
Notes
The four-minute difference is why the night sky changes through the year and why a given constellation rises four minutes earlier each night, or two hours earlier each month. Observatories run on sidereal time because a telescope pointed at a fixed sidereal time and position finds the same object every night.