by Jack St. Louis
The Meridian is an imaginary line, part of a “Great Circle” running from the North Celestial Pole, through the point directly overhead – the Zenith – continuing to the South Celestial Pole, through the point directly under your feet – the Nadir – and back to the NCP. We are only concerned, of course, with the ‘visible’ portion of the Meridian above the horizon.
Why? When any celestial object crosses the Meridian, that is the time & location when the object is highest in the sky. At that time, you are looking through the least amount of atmosphere and so have the best view of the object. Viewing areas close to either side of the Meridian won’t be noticeably worse, but technically, along the Meridian is the best viewing.
Using a computer planetarium program or a Planisphere, also called a Star & Planet Finder, can help you plan when to view the objects you want to see along the Meridian. On your program, like the free on-line Stellarium, you can set the Meridian line On and even select the color of the Meridian for prominence. On some planispheres, there are two small circles where you can punch holes and place a string to mark the Meridian.
The good thing is everyone has their own Meridian, and it follows you wherever you go, you are never without it. Also, your Zenith and Nadir are always with you.
The term ‘culmination’ is used to specify when a star or deep sky object is on the Meridian. When culmination occurs at midnight, that object is said to be at ‘opposition’, so all the stars, nebulae, clusters and galaxies have an opposition date & time.
Using Stellarium, and setting the date to April 15th and the time to 9:00 PM, (21:00:00 – auto adjusts to UTC-4:00 for EDT), I find Alphard, in Hydra, past the Meridian, approaching culmination is Regulus, in Leo. Polaris is always close to culmination. Advancing the time to place Regulus on the Meridian gives a time of 9:24 PM (21:24) Setting Jupiter on the Meridian gives a time of 00:20, 20 minutes past midnight. Not sure if I can stay up that late.
A Planisphere is even easier, simply place your object under the Meridian string on the night you plan to observe and check the time. Say on May 15th I want to view M51, the Whirlpool Galaxy in Ursa Major, and have the best view to sketch or image it; putting M51 on the Meridian gives an approximate time of 10:30 PM I can stay up that late. The difference between EST & EDT will not make much difference in the view.
If you look North, on the section of the Meridian between Polaris and the northern horizon, objects will be at ‘lowest culmination’, or the lowest altitude the circumpolar objects will be. So, circumpolar objects have two culminations, high & low.