Q&A: To See the Southern Cross

Question: Now that I’ve moved from Phoenix to San Antonio I can see a little farther south in the sky. I recall an article you wrote about the near-impossibility of seeing the Southern Cross from Phoenix, and wondered if I was now far enough south to see it?  — Richard Searles, San Antonio, TX

Answer: Nice to hear from you Richard. [We were in the same astronomy club back in the 80s-90s.] A copy of that old article has been emailed to you. For other readers interested in seeing it, click here (106k PDF). It was written when I was using a much less powerful computer, so please excuse the crude graphics.

Normally, the best view of the Southern Cross is from the southern hemisphere. But from southern parts of the northern hemisphere, part or all of this famous constellation can be seen. This somewhat lengthy post is your “how to” guide. It will demonstrate how to find, from any location, just how far south you can see on the celestial sphere — the globe of stars surrounding us in space.

I have an easy formula for calculating that. It’s based on high school geometry. From San Antonio, at latitude 29° 25′ N (about 4° farther south than Phoenix), you can theoretically see as far south as declination δ = -60.6°. The formula is simply: δ = latitude – 90°. Click on the thumbnail below for details.

The Southern Cross is officially known as the constellation Crux (Latin for “cross” — rhymes with “flux”). The northernmost star in that bright constellation is γ Crucis, a red giant. Its distance is 88.6 light years, and its apparent magnitude is +1.63, making it the 25th brightest star in the sky.

The γ (gamma) tells us it’s the 3rd brightest star in that constellation, after α (alpha) and β (beta) Crucis. The genitive form of “Crux” is “Crucis” and that’s how astronomers label most bright stars.

The declination of γ Crucis is δ = -57°, and based on the geometry in the thumbnail, its maximum elevation above your horizon in San Antonio will be 3.4°. That’s still pretty low, so to see γ Crucis, you’ll need to optimize several factors:

  • First, and most obviously, you’ll need to catch γ Crucis when it’s highest in the sky, a position known as culmination. That happens when it crosses the Celestial Meridian, located due south for northern hemisphereans. More later about how to figure out when culmination occurs.
  • Second, let’s choose an observing time of 1:00 am. By then, most outside lights and billboards have been turned off, and the sky will be at its darkest for that 24 hour period.
  • Third, to whatever extent possible, observe from south of the San Antonio metropolitan area. If you observe through the haze and light dome over the city, your chance of spotting γ Crucis will be significantly reduced. With an elevation of only 3.4°, it’s magnitude will be reduced to +3–4 (depending on atmospheric transparency).
  • Finally, you’ll need an unobstructed view of the southern horizon, with no mountains, trees, or buildings in the way.

Do all that, and you will have established the optimum geometry for observing γ Crucis. But even then, there’s the vagaries of weather, so pray for clear sky down to your southern horizon.

The time between rising and setting for γ Crucis is only 4 hours 14 minutes. It will rise at 22:51 (the preceding day) near azimuth 165°, and set at 03:05 near azimuth 195°. Culmination will occur at our specified 01:00.

And that brings me to HOW we tell when an object of interest is at culmination. In the past, we had to compute the culmination numbers by hand, using the formulae provided in such classics as Practical Astronomy With Your Calculator (and later for spreadsheet). If you really want to do these laborious calculations, you can also find the formulae online at websites like this. But there’s a much easier (and entertaining) way to get the same answers.

Download any of the free apps that bill themselves as a virtual planetarium. I especially like Sky Chart. It’s designed for use on a desktop with a large screen, but will run on laptops and some tablets. Been using it for about 6 months and really like the UI and settings options. The help files are pretty good too. From this point on, I’ll assume you have your preferred app and know how to use it. I’ll keep the instructions generic.

The graphic at top shows γ Crucis at the instant of culmination as seen from San Antonio on April 8, 2014 at 01:00 am. It’s a screen capture of what Sky Chart displayed. Here’s how I got there …

  • Look up the declination of the target object so you know where to watch on the screen. The display will have curves of declination (or elevation) superimposed over the stars.
  • Input the latitude, longitude, and altitude of the location from which you’ll be observing.
  • Change your direction of view to the south (azimuth 180°).
  • Set the zoom, or field of view (FOV) to 45-90° width as a starting point. Zoom-in later as needed.
  • Set the time to whenever you want to observe. For fainter objects, use 1:00 am as I did.
  • Set the time increment to 1 week. 10-day increments also work well.
  • Start incrementing the time forward in 1 week steps, watching for your target object to appear. You’ll see the constellations marching across your screen from east to west (v = 1°/day). When your object comes into view …
  • reduce the increment to 1 day. Continue incrementing forward and backward until you have the desired object as close to culmination as you can get. The time should still read 01:00.
  • Note the data displayed. If the time is not 01:00, a daylight saving time adjustment occurred. How different apps handle daylight saving time (DST) varies. You can usually find the settings somewhere in the options.
  • Add that date to your calendar, and happy hunting. May your skies be clear.

This is only one example of how you could use a virtual planetarium. Their potential is virtually unlimited (pun intended). You could, for example: See what the sky looked like on the day you were born, or identify that bright object next to the Moon, or watch how the sky changes in 1 century increments, or see what Mars actually does when it goes into retrograde motion, or predict the next time the Moon will occult Jupiter as seen from the South Pole.

On a portable tablet, these apps are also a great way to just learn your way around the sky, seeing the names of stars, planets, and constellations displayed onscreen. Using GPS and inertial sensors, the app will show onscreen whatever you have the tablet pointed at.

The most southerly object I’ve seen from Phoenix is Canopus (δ = -52° 42′). I had to compute its culmination numbers the long way, using a scientific calculator. Never again. These days I let my computer do all the number crunching. Leaves a lot more time for actual observing.

Next Week in Sky Lights ⇒ 2014 Conjunction of Mars and Antares

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2014 Conjunction of Mars and Antares