Did you ever wonder why each of the seasons feature a unique and distinctive procession of stars and constellations?
The year is divided into four seasons, Spring, Summer, Autumn and Winter. We begin the astronomical year on the Vernal Equinox, the astronomical beginning of Spring and the date the sun, moving eastward along the Ecliptic, crosses the Celestial Equator. In this article we will discuss the changing canopy of stars above with the change of the seasons.
Seasonal change is one of the first things we learn about and notice as children, observing deeper, more subtle changes as we learn more and expand our horizons. In the simplest sense, we mark the passage of the seasons by changes in the climate, a change driven by the changing angle of the sun’s energy as it impinges on the earth’s surface (see illustration below). As we grow and learn, we begin to observe these subtle changes such as the migratory patterns of birds and the changing flora and fauna, both also driven by the changing climate; in a separate article, we will discuss the why and wherefore of this change.
We know by now that the Earth is the third planet from the sun with a period of 365 days, moving approximately one degree in its orbit per day (360/365 ~1). The Earth also rotates once every 24 hours, the period known as a “Solar Day”. On only two days of the year, the Vernal Equinox and its Autumnal analog, the Autumnal Equinox, the astronomical beginning of autumn, the “solar day” is divided into two equal, 12-hour periods and thus, we experience equal hours of daylight and darkness on those dates.
Author’s note: This year’s Vernal Equinox occurs on Tuesday, March 20, 2018 at 16:15 UTC (12:15 PM, EDT)
As passengers on this beautiful blue planet, we look up and out at the stars and the universe around us, keeping in mind that the earth is moving and, as it does, our view of the sky would necessarily change as any background would change for a moving observer; consider the changing background one observes while on an amusement park ride, a good example and analog for this discussion.
One must also remember that our star and solar system are part of a huge agglomeration of stars known as the Milky Way galaxy, a huge collection of stars, gas and dust with the number of stars in the hundreds of billions. Our galaxy is a vast, rotating disk known as a Spiral Galaxy with our sun and solar system located almost precisely on a plane that bisects the galactic disk. When we look up and observe the Milky Way, the beautiful band of light above, we are looking across this plane, out through the galactic disk.
Our Milky Way galaxy as it would appear from above. Note the yellow path, outlining the sun’s orbit around the galactic center. Image credit NASA/JPL/GSFC
During the Spring and Autumn the Earth affords us a view away from the plane of the Milky Way and thus the sky may seem a bit more sparse; perhaps so, but each of the seasons’ sky has its own, unique qualities and character. Since we are not looking through the Milky Way’s galactic plane during these times of the year, we are able to observe more galaxies, great stellar systems external to our Milky Way that would otherwise be hidden by the milky way’s gas and dust. It also goes without saying that each of these seasons present with a unique set of constellations. For example, during the spring, two prominent constellations are Leo and Virgo, both constellations of the Zodiac and both rich with galaxies and clusters of galaxies. A prominent Autumn constellation, Andromeda, contains the great spiral galaxy Messier-31 (M-31), in Andromeda, the galaxy famously observed by Edwin Hubble in his landmark discovery that the universe is expanding and that galaxies, like the Milky Way, are vast, separate systems of stars unto themselves.
The great Spiral Galaxy in Andromeda, Messier-31. Image credit Robert Gendler, NASA, STSci.edu
The summer and winter skies each possess a unique and distinctive character, a character based on a view of the galaxy of stars in which we live, a view of the night sky afforded by the Earth’s position in its orbit around the sun. While each of these opposite seasons contain their share of bright stars, the respective seasonal constellations and the view of the milky way are quite different. The summer sky presents with the rich, gossamer beauty of the Milky Way’s galactic center located to the south in Sagittarius and to the west towards Scorpio for Northern Hemisphere observers with myriad star clusters and beautiful, glowing nebulae. During the winter months, the earth, in its orbit around the sun, affords us a view directly opposite that of the summer, opposite the galactic center and towards the Orion-Cygnus arm of the galaxy (see illustration above). The winter milky way is thus more subtle than the summer milky way but nonetheless has its own unique beauty. A dark winter sky will allow us to see the winter milky way running north through Canis Major and Orion, up towards the zenith and continuing through Cassiopeia and Perseus.
On the next clear day, go outside before sunset, take note of how your surroundings have changed, watch the sun as it sets and then look up at the starry night sky. If you do this two or three times a week, you will come to learn more about our world and our universe than can be learned through reading a thousand textbooks. If you continue to do this beginning now, over the course of the next three months into June, you will see a whole new set of stars and constellations by the time the Summer Solstice is upon us on Thursday, June 21, 2018 at 10:07 UTC (6:07 AM, EDT).
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