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. 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
| 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 |
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