| A - SPECIFIC |
|---|
Use "a" to refer to one you observe or have knowledge about. |
A comet was seen in the western sky last night. (first mention) |
The comet was believed to be Hale-Bopp. |
The comet has a spectacular, bright tail.
|
| A - UNSPECIFIC |
|---|
Use "a" to talk about one in general (one as an example of all; true of all comets). |
A comet appears to have a tail as it streaks through the sky. |
A comet also has a dense head. |
If you look up in the sky, you might see a comet. |

Amazing Space: Online Explorations (NASA) http://amazing-space.stsci.edu
"Comets Facts, Myths and Legends" Amazing Space, 2011, http://amazing-space.stsci.edu/resources/explorations/cometmyth/home.html