When is the best time to see a planet or constellation?
When a planet or star is crossing the meridian it is at the highest altitude it reaches in the sky, and therefore at its farthest from dust and horizon haze. This is called its meridian transit or culmination. The best time to see a star or planet, then, would be around its culmination.
When a planet or star is transiting the meridian at midnight it is opposite the Sun, or "in opposition." If it is high enough, a planet or star at "opposition" may be visible all night long:
- Rises in the east when the Sun sets in the west.
- Transits the meridian at midnight.
- Sets in the west when the Sun rises in the east.
Test your understanding:
- What is the phase of the Moon when it is in opposition?
Where should I look for a planet or star after its midnight culmination?
If its altitude on the meridian is high enough, a given constellation may be visible in the early evening sky for two or three months after the date of its midnight culmination.
After the date of its midnight culmination it will transit the meridian at an ever earlier time each evening, until eventually it will already be in the western sky when it first appears at sunset.
Finally, it will set with the sun and enter the daylight sky.
Try to identify the constellations listed for each month of the year! Before midnight, look for each month's constellations rising higher in the eastern sky. After midnight, look for them setting in the western sky.
Before midnight, look for the previous month's constellations setting in the western sky.
For nothern hemisphere observers, the most southerly constellations may only be visible at their midnight culmination, if they never get very high in the sky.
Use a planisphere to locate any constellation visible at any time of night on any day of the year.
Trick question: When might a meridian transit not be the same as culmination?
- Answer: Circumpolar stars cross the meridian twice each day without setting below the horizon. Only the higher transit of the two would be a culmination.