Huestis: “March”-ing into Spring Saying Goodbye to the Winter Sky

Wednesday, February 28, 2018

David Huestis, GoLocalWorcester Astronomy Expert

March begins with a Full Moon on the 1st and ends with a Full Moon on the 31st. Therefore, like January, the second one is called a Blue Moon. Regrettably there is no lunar eclipse this time.

While we are still a couple of months away from the return of Jupiter to the early evening sky during public observing sessions at the local observatories, March does provide us an opportunity to observe our solar system’s two innermost planets to the Sun. I’m talking about Venus and Mercury.

At the beginning of March these planets will be visible in evening twilight just above the western horizon after sunset. Venus will be the brighter of the two objects. On the 3rd Mercury will be about one degree (two full moon diameters) to the right of Venus. Each evening they will rise higher into the sky, though Mercury will very noticeably increase its separation from Venus rather quickly as they do so. On the evenings of the 16th and 17th Mercury will be at its highest elevation above the horizon. Then on the 18th a waxing crescent Moon will complement the Venus and Mercury sky scene. Each evening thereafter Mercury will begin descending the sky towards the horizon and will soon be lost in bright twilight. Venus will continue its ascent into the sky as the month progresses.

Through a telescope Venus will appear almost fully illuminated like a full moon for the entire month. While Mercury will start out nearly in full phase as March begins, its phase will quickly change as it dips back towards the horizon and the Sun. By the 22nd, when Mercury will once again be to the right of Venus, his phase will look like that of a waxing crescent Moon.

An important date to remember is Sunday, March 11. This day is when most of the United States set clocks ahead one hour to Eastern Daylight Time (EDT) at 2:00am. This annual ritual is known as Daylight Saving Time. Don’t forget to do so or you’ll be late for any Sunday morning function.

Another important day is March 20. At 12:15pm the vernal equinox (equal day and night) occurs. Spring begins in the northern hemisphere. Since the Winter Solstice observance back on December 21 the Sun has been steadily moving northward in our sky. On the vernal equinox if you were standing at a location on the Earth’s equator the Sun would be directly overhead (zenith) at local noon. We look forward to longer daylight hours and warmer temperatures.

However, it is not too late to say goodbye to the winter constellations. It was so cold and windy this past winter that I did not spend much time observing some of the brightest and most recognizable star patterns in the sky. In fact, the winter sky contains seven of the 23 brightest stars we can see from the Earth.

There is a huge winter sky asterism formed by combining some of the brightest stars of six constellations. It’s called the Winter Circle or Winter Hexagon. Please reference the accompanying star map. Betelgeuse, though inside either pattern, is still considered part of the asterism.

Before we examine each of the stars in the Winter Circle, let’s review three important terms. First, the brightness of any celestial object is called its magnitude. The basic idea is that the more negative the magnitude, the brighter the object. The more positive the magnitude, the dimmer the object is. So the Sun is -26.74, the Full Moon -12.92, Venus -4.89, Saturn approximately 0, well known Polaris (the North Star) is magnitude +2, and the naked-eye limit with no light pollution is magnitude +6. Pluto is about +13.65. (Usually the plus sign (+) is assumed and not used, but I do so in this column for clarity.)

Second, a star’s distance is measured in light years. One light year is equal to just less than six trillion miles. Third, the spectral classification of a star is defined using the following letters: O, B, A, F, G, K, or M, and often followed by additional numbers and letters to further refine the classification. “O” stars are the hottest while “M” stars are the coolest.

Let’s start our tour of the Winter Circle with the brightest star we can see in the sky (besides the Sun of course) — Sirius. Sirius is in Canis Major, the Big Dog. Sirius shines at magnitude -1.44 and it is 8.7 light years away. Do the math and this fairly close neighbor to our Sun is 52.2 trillion miles from us. For you Rhode Islanders that’s much farther than Woonsocket or Westerly! Sirius is a hot, blue-white star (spectral class A0) about 1.7 times the diameter of our Sun.

Next we move northward and clockwise in the sky to locate Procyon in Canis Minor, the Little Dog. Procyon is a white star (F5) shining at magnitude +0.40 and is 11 light years distant. It’s about twice the diameter of our Sun. Moving farther northward we encounter the Gemini twins, Pollux and Castor. Pollux is 34 light years distant, while Castor is 18 light years farther away at 52. Pollux is a cool, orange giant (K0) ten times the Sun’s diameter, while Castor is a hot, blue-white star (A1) only twice the diameter of the Sun. Pollux and Castor shine at +1.16 and +1.93 magnitude respectively.

Now we swing up and over to a constellation almost directly overhead — Auriga, where we find +1.93 magnitude Capella. While Capella (G6) is a class “G”-type yellow star like the Sun (G2), it has three times more mass and is just over seven times the Sun’s diameter. Next we proceed south to encounter the orange giant (K5) Aldebaran in Taurus. Aldebaran represents the bull’s eye in the star pattern known as the Hyades star cluster (shaped like a “V”). Aldebaran, 65 light years away, is a cool star which has expanded to be just over 44 times the diameter of the Sun with only 2.5 times our Sun’s mass.

Continue to swing southward in the sky until we arrive at the bottom right star representing Orion’s left foot. (Please note: Orion is facing us.) This star is +0.18 magnitude Rigel, a blue supergiant (B8) 800 light years away — the most distant of the Winter Circle stars. Rigel is 62 times the diameter of our Sun and contains 17 times more mass. We now complete the tour of the Winter Circle by swinging back to Sirius.

But wait. No, I didn’t forget about Betelgeuse. Betelgeuse is the red supergiant (M2) star that marks the top right shoulder of Orion. It shines at magnitude +0.45 and resides at a distance of 520 light years. Betelgeuse is also a very large star, measuring in at a conservative 950 solar diameters. If you replaced our Sun with Betelgeuse it would extend out to the asteroid belt between Mars and Jupiter.

As you can see by this small sampling of stars that comprise the Winter Circle, stars are quite a lot like people. They are all different, but their differences make them unique and important.

The next time you have an opportunity to observe the Winter Circle, you will have a better understanding and appreciation of the scale and diversity of our stellar neighbors in this region of the Milky Way Galaxy.

While you do not require a telescope to appreciate the magnificence of the bright stars comprising the Winter Circle, when Jupiter, Mars and Saturn return to the mid-evening sky the views of these planets from the local observatories and their fine instruments will reward you with incredible images. Seagrave Memorial Observatory in North Scituate is open every clear Saturday night. Ladd Observatory in Providence is open every clear Tuesday night. The Margaret M. Jacoby Observatory at the CCRI Knight Campus in Warwick is open every clear Thursday night. Frosty Drew Observatory in Charlestown is open every clear Friday night.

Keep your eyes to the skies.

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Huestis: “March”-ing into Spring Saying Goodbye to the Winter Sky

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