Sky Talk May 2009: The Amazing Rotating Star-Wheel

The Scientifics Star and Planet Locator (often referred to as the “rotating star-wheel” since it consists of a round star chart contained within a square housing) is one of the simplest, least expensive, and yet most useful devices ever offered beginning stargazers. It’s a compass, calendar, clock, celestial roadmap, sky previewer and planet finder all wrapped into one!

One of the first meager purchases I ever made as a budding 12-year-old stargazer was that of a rotating star wheel. This nifty device consists of a wheel with a star chart printed on it, along with the days and months of the year appearing around its periphery. It’s housed within a square enclosure with cutouts in it including a large oval one exposing part of the star chart. Each of its corners is marked with the cardinal points, and the hours of the day and night are shown in a circle within it. To use it, you simply set the date you’re observing opposite the time by turning the star wheel. Holding the device over your head and pointing the north corner towards the north, it then shows the sky as it currently appears.

The Locator (#30092-27) is obviously a compass, since pointing the corner north as we just did then indicates the other directions in the sky. And it’s a handy star chart that shows the major constellations and brightest stars (names of which are clearly labeled). It even indicates the flow of the Milky Way from horizon to horizon, and the pathway of both the ecliptic and the celestial equator. But what about its being a clock and a calendar? Here’s where a clever technique involving “role reversal” comes into play.

Let’s assume that you’re outside on a clear night with your trusty Locator. You know what date it is but not the time (since you’re not wearing a watch). Rotating the star chart, orient it to match the sky exactly as seen above you. (The better you know the constellations, the easier this becomes.) Once the chart appears to show what you’re seeing, look opposite the date and you will find the time as indicated on the clock face around the star wheel’s housing! Conversely, let’s say you’re wearing a watch and know what time it is but not sure about the date. Again, rotate the wheel to carefully match the sky overhead. Look at the time, and then find the date as it appears on the periphery of the wheel!

And there are still other uses for the Locator. Even though it’s late Spring as you read this, maybe you would like to preview what the sky will look like on Christmas. Simply set the star wheel for December 25th at, say, 9 p.m. and there it is. Or perhaps you’ve gone outside just before dawn this month on the way to work and noticed a brilliant object in the eastern sky that’s not on the star chart. Chances are that its one of the bright naked-eye planets. Since these wanderers are constantly changing position in the sky, they’re not plotted on the star wheel itself. But turn the Locator over and there on the back side is a table of planetary positions by month and year. Looking at May 2009, you find that indeed Venus is in the morning sky. Yes, the Scientifics “rotating star wheel” is truly an amazing device!

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of five books on stargazing.

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