天底

tiāndǐ
(astronomy) nadir

CEDICT

astronomy

Entry Composition

1 天 tiān
  1. day
  2. sky
  3. heaven
2 底 dǐ
  1. bottom; base
  2. background; foundation
  3. copy (of a manuscript or receipt etc) kept as a record
  4. (suffix) end of a year or month
  5. (math.) radix; base (abbr. for 底数 dǐshù)