The Six-inch Transit Circle has been in operation for 100 years, observing stars and solar system objects fundamentally from the same location in Washington, DC. It measures the "constants" of azimuth, collimation and nadir and level and makes observations of the Sun and planets. The fundamental azimuth is determined from observations of circumpolar stars. A brief explanation of each catalog has been provided by the Astronomical Data Center.
The star catalogs observed with this instrument and available here are:
| 1. | W10 | 2499 stars |
| 2. | W25 | 2383 stars |
| 3. | W150 | 1536 stars |
| 4. | W250 | 5216 stars |
| 5. | W350 | 5965 stars |
| 6. | W450 | 2685 stars |
| 7. | W550 | 4916 stars |
| 8. | W1J00 | 7267 stars |
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