| The period of time before sunrise or after sunset where the midpoint of the sun disk is 6 degrees or less below the true horizon. |
| The period of time before sunrise or after sunset where the midpoint of the sun disk is 6 degrees or less below the true horizon. |
| The period of time before sunrise or after sunset where the midpoint of the sun disk is 12 degrees or less below the true horizon. |
| The period of time before sunrise or after sunset where the midpoint of the sun disk is 12 degrees or less below the apparent horizon. |
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