Seasonal Movements of the Milky Way Core (Patreon)

Although the core of the Milky Way (MW) moves nightly from East to West at about 15 degrees per hour, the seasonal movement adds another 30 degrees per month —moving the MW almost 150 degrees to the West, going from April to September. You can forecast your MW alignments, with foreground objects, using apps like PhotoPills, Planit Pro and Sky Guide. (Naturally, this presumes you live in the Northern Hemisphere. If you live in the Southern Hemisphere, or are traveling there, you'll need to flip flop these directions and dates. I'll give more on the Southern Hemisphere positions and movements in another post.)

In general terms, the MW Galactic Core (the brightest area), rises in the Spring time, in a somewhat horizontal position that is quite parallel to the horizon. The core clears the horizon in the early morning hours of the night, or from about midnight to 3:00 A.M., for those living near the 40th Parallel. The earlier in the Spring, the later in the morning it takes to clear the horizon. These times and positions of the core are for when the MW is most visible, or during the Astronomical Dusk (AD) hours, when the night sky is at its darkest.

By Summer, the MW core starts to rise earlier during the Astronomical Dusk period (rising above the horizon between 10:00 P.M. and 11:30 P.M). It starts out in a diagonal position to the horizon, and by midnight it is almost perpendicular to the horizon. Between June and July, the core is at its highest point of the year, above the horizon.

When Autumn arrives, the MW core starts rising even earlier during the AD period (rising above the horizon between 8:00 P.M. and 9:30 P.M). The core only makes its appearance above the horizon for a couple of hours in late September, and by November, it can often be less than a half-hour.  The position of the MW just above the core starts out almost perpendicular, and by 9:30 to 11:30 P.M. it begins to lean to the right, with the core sinking below the horizon.

The photo on the right is an image my friend, Robert McKendrick, took in the Spring (mid April, at about 2:30 A.M.) of a butte located in southern Utah. The Google map on the left show the approximate camera angle and position of the Milky Way core.

The photo on the left is also by Robert, taken of the same butte, only this time in the Autumn (early October). The time is about 10:30 P.M., and already the core is well below the horizon, and the MW above the core is leaning to the right. (The green color of the sky is due to a high level of ''airglow'' in the sky that night.)

Both of these photo were taken during or in preparation for a Remote Utah Workshop that Robert and I conducted this Fall. The three images below show how we PLANNED these photos using the Planner tools in PhotoPills.

KEY: The yellow and orange lines show the direction of the rising and setting sun. The light blue and dark blue lines show the direction of the rising and setting moon (all of these positions are irrelevant for this discussion). The gray and dark gray lines shows when the Astronomical Dusk period begins and ends. The thin white line show the positions of the MW where it intersects the horizon. The thick white line shows the direction for the core of the MW at the planned time. The dotted arc show the overhead orientation of the MW, and the largest white dot indicates the position of the core at the planned time.

1. This is the plan for the Spring photo Robert shot in April. Note how the dotted arc leans far the the right, indicating that the positioning of the MW is quite diagonal to the horizon. The top bar show the times of Visibility for the GC (Galactic Core), with the core being visible above the horizon (during the AD period) for almost four hours. By mid-June, that period will increase by at least an hour, and then begin to decrease as Summer wanes into Autumn.

2. The middle plan is for the Autumn photo Robert shot in the first week of October. Our plan was to take the photographs around 9:30 P.M. so that the position of the MW would be almost perpendicular (note that the dotted arc is almost directly over the white line), with some of the core still above the horizon. You'll also notice that the total time of Visibility for the GC is now down to less than two hours. (From about late November through late February, that GC Visibility period will be zero for both the Northern and Southern Hemispheres.)

3. By the time Robert and I helped all our workshop attendees get their shots, it was almost 11:30 P.M. The core was now well below the horizon. Visibility ended at 10:24 P.M. (note that there is no longer a thick white line, pointing to the direction of the core), and the position of the MW, above the core, was leaning to the right (indicated by a greater arch to the dotted arc).