The Sky That Nearly Stands Still

Why do some constellations linger, while others quickly exit the celestial stage?

Star map

Brilliant Arcturus (the orange star near the centre of this chart) is a presence in the evening sky from spring through to late autumn. Cartography by Glenn LeDrew

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.

Categories: Stars and Constellations
3 comments on “The Sky That Nearly Stands Still
  1. Jim Catton says:

    Of course they hang around,they are circumpolar and don;t make as big a sweep of the sky as a star in lower latitudes.

  2. It’s interesting to read about the experience of frustration every fall for the evening skies. If there are clear skies in the morning I like to get up early. And for the same reasons as explained by Mr. Rodger the skies seem to “accelerate”. Middle of July one can hardly see Orion, now it is quite prominent almost in the southern skies. And Sirius becomes visible quite fast in the second half of August.

    Best regards

    Gerhard Salhenegger

  3. David A. Rodger says:

    JIm Catton. Being circumpolar does ensure that a star or constellation remains visible every night. I agree. But Arcturus isn’t circumpolar at mid-Canadian latitudes, nor are Virgo or Libra, and Vega and Deneb are barely circumpolar. The main factor in the phenomenon I’m describing is the progression of earlier sunsets in fall. If you start an observing session at nightfall, as I usually do, the effect is obvious.

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