S&T photoillustration by Steven Simpson and Akira Fujii.
One of the challenges of publishing an astronomy magazine for a global readership is that we don't all see the same sky. Most obvious are the different constellations visible from the middle latitudes of the Northern and Southern Hemispheres, where most people live. This is a consequence of our living on a giant ball of rock. Relative to the stars, "up" for me in Massachusetts is "down" for a fellow astronomer in southern Chile. Although we're in the same time zone and might both be using our telescopes at the same moment, the stars over my head are below my counterpart's horizon, and vice versa. That's why we print two evening sky charts each month, one for 40° north latitude and another for 35° south.
Each issue of Sky & Telescope magazine contains a foldout chart of the Northern Hemisphere's evening sky for that month. It is computed for an observer at 40° north latitude.
There's a further complication caused by Earth's daily spin: the constellations visible from a given location are constantly changing, with new ones rising in the east as others set in the west. That's why we print dates and times on our star charts. Finally, the naked-eye planets don't stay put. This is especially true of Venus, which moves across a large swath of sky each month. We compromise by plotting the position of the planets for midmonth.
The bottom line is that unless you live at exactly 40° north or 35° south, and unless you restrict your stargazing to midmonth at exactly the time for which your hemisphere's chart is plotted, what you see in the sky won't quite match what's on the map. Veteran amateur astronomers know this instinctively and make mental adjustments with little effort. But beginners do not, and trying to learn the constellations with charts that don't match the sky faithfully can be extremely frustrating.
