mid-15c., "capable of being considered, conceivable," from Medieval Latin considerabilis "worthy to be considered," from Latin considerare "to look at closely, observe," probably literally "to observe the stars," from assimilated form of com "with, together" (see con-) + sidus (genitive sideris) "heavenly body, star, constellation" (see sidereal).
Meaning "pretty large" is from 1650s (implied in considerably), from now-archaic earlier sense of "Worthy of regard or attention" (1610s).
CONSIDERABLE. This word is still frequently used in the manner pointed out by Dr. Witherspoon in the following remark: "He is considerable of a surveyor; considerable of it may be found in the country. This manner of speaking prevails in the northern parts." [Pickering, "A Vocabulary, or Collection of Words and Phrases Which Have Been Supposed to be Peculiar to the United States of America," 1816]
1570s, "marked by deliberation," from Latin consideratus, past participle of considerare "to look at closely, observe" (see consider). Of persons, "deliberate, prudent, given to consideration" 1580s; meaning "not unfeeling or rigorous, showing consideration for the circumstances or feelings of others" is from c. 1700. Related: Considerately; considerateness.
"cautious, wary," literally "looking about on all sides," early 15c., from Latin circumspectus "deliberate, guarded, well-considered," past participle of circumspicere "look around, take heed," from circum "around, round about" (see circum-) + specere "to look" (from PIE root *spek- "to observe"). Related: Circumspectly; circumspectness.
"sense-perception," 1833 as a German word in English, nativized from 1848, from German Anschauung "mode of view," literally "a looking at," from anschauen "to look at," from Middle High German aneschouwen, from an (see on) + Old High German scouwon "to look at" (from PIE root *keu- "to see, observe, perceive"). A term in Kantian philosophy.
"building for observing astronomical phenomena," 1670s (in reference to Greenwich), from French observatoire, from observer (v.) "to observe, watch over, follow," from Latin observare "watch over," from ob "in front of, before" (see ob-) + servare "to watch, keep safe," from PIE root *ser- (1) "to protect."