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A half-month is a unit of time typically used in astronomy. Each month is separated into two parts:

  • days 1 to 15
  • day 16 until the end of the month.

Unidentified celestial objects, such as comets and asteroids, are given designations that contain the half-month as a letter of the English alphabet.[1][2] For example, an object seen in the second half of January would be identified with the letter B; if seen in the first half of February, the letter would be C; the letter I is not used, to prevent confusion with the number 1. Instead, the letters proceed from H (April 16–30) to J (May 1–15).

Half-month letters[1]
Letter Dates
A January 1–15
B January 16–31
C February 1–15
D February 16–29
E March 1–15
F March 16–31
G April 1–15
H April 16–30
Letter Dates
J May 1–15
K May 16–31
L June 1–15
M June 16–30
N July 1–15
O July 16–31
P August 1–15
Q August 16–31
Letter Dates
R September 1–15
S September 16–30
T October 1–15
U October 16–31
V November 1–15
W November 16–30
X December 1–15
Y December 16–31


  1. ^ a b "New- And Old-Style Minor Planet Designations". Minor Planet Center. Retrieved 17 December 2018.
  2. ^ "Naming of Astronomical Objects". International Astronomical Union. Retrieved 17 December 2018.

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