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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Sliding south to Spica!

Prime time in May finds the Big Dipper high in the north east. Take a slide down its gracefully curved handle, first to  our April guidepost star – Arcturus – and then on to Spica, our May guidepost star, which happens to be  as close to first magnitude as any star in our sky.

If you thought all our guidepost stars were first magnitude, think again. Technically, to be called “first magnitude” a star should be in the range of .5 to 1.5.  But to be a “guidepost” star we use a broader definition. The star must be very bright and help us anchor an important section of sky. So we call Polaris a “guidepost,” yet it is almost exactly magnitude 2. But being almost at the north celestial pole, it certainly is a useful “anchor” for us. And, of course, we call  Sirius a “guidepost” star and it is much brighter than first magnitude at -1.46. In fact, several of the guidepost stars are brighter than first magnitude, including Arcturus which is -.07. .Spica, though is right on target. It’s official magnitude is .96, so it’s as good a representative of a first magnitude star as any. Look at it – then look at Polaris. Spica is about 2.5 times as bright as Polaris – that’s how much difference there is between magnitudes. OK – a difference of one magnitude is actually 2.512. Why? read “Step 6 – How bright is that star?”   for a complete explanation.

A couple notes about the chart below. First, it is adapted from Starry Nights Pro software and when showing this much sky, the horizon is represented as curved upward. This may catch you unawares, for Spica is closer to the horizon than it may look at first glance. Arcturus is about 52 degrees above the eastern horizon while Spica is nearly 20 degrees lower in the southeast.

Second, Alkaid, the last star in the handle of the Dipper, is named because it too is a second magnitude star like Polaris, though Alkaid is a tad brighter than Polaris. Still, you get a good representation of the magnitude system by looking at these three stars – Alkaid, Arcturus, and Spica, since they are magnitude 2, magnitude 0, and magnitude 1 respectively. Once you have located Spica, study the three stars with that in mind – but at the same time, don’t forget that any star lower int he sky will look a bit dimmer than it should simply because you’re looking through a lot more atmosphere than you are when looking at a star that is high.


Start your slide on the handle of the Big Dipper, continue down and past Arcturus, and down more to Spica. (Click on image for larger version.)

One Response

  1. […] months, compare it to the icy blue of Spica, now in the southwest sky. (Remember – you reach Spica by following the arc of the Big Dipper’s handle first to Arcturus, then continuing on to […]

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