It’s a frustration we experience every autumn—the relentless westward march of the seasonal constellations seems to slow to a crawl as we pass from late July through early November. Look west at nightfall in mid-September, for example, and you’ll find Arcturus, with the Big Dipper high in the northwest. Look again in October and, well, there’ll be Arcturus, with the Big Dipper high in the northwest. The scene appears to remain static even into November.
What’s going on here? We know that the stars advance westward by approximately four minutes each night (3 minutes, 56 seconds to be precise), which totals about half an hour a week, and two hours a month. In other words, a given star crosses the meridian (the line that joins north to south and passes directly overhead) nearly four minutes earlier each night or two hours earlier each month. That never varies, regardless of the season. It’s a function of our annual journey around the sun, and reflects the tiny additional bit of rotation our planet has to make to ensure we can keep time by the position of the Sun, and not the stars.
Fighting against the four-minutes-a-day rule is the changing time of nightfall. After the summer solstice, darkness arrives earlier each night—and it does so almost fast enough to keep pace with the westward motion of the constellations.
Let’s take Vancouver, British Columbia, as an example. As darkness falls on the summer solstice (at around 11 p.m., PDT), Arcturus hangs some 54 degrees above the northwest horizon. By the autumn equinox, it gets dark a little after 8 p.m., but Arcturus is still 25 degrees high—it’s lost less than 20 degrees of elevation in three months! Yes, the constellations have moved westward, but not nearly as much as you would expect after a full season has passed. As a result, constellations like Cygnus, Lyra and Boötes (home to Arcturus) seem to hang around forever.
These same factors come into play during the spring, too, but in reverse. The stars continue their relentless march westward at four-minutes per night, but now twilight occurs later and later. In effect, the extra daylight swallows up the advancing winter constellations. That’s why Orion, Taurus, Gemini and Canis Major seem to disappear from the celestial stage so quickly.
David Rodger, the founding director of Vancouver’s HR MacMillan Planetarium, has been explaining astronomy to the general public in person, in print, in planetariums and observatories and on television, for almost 60 years.