March Sky Chart; Northern Hemisphere Gazer's guide

By , Director of the American Museum of Natural History, New York

March 1: Mercury is stationary among the stars and resumes its direct (easterly) motion. The planet is in the morning sky, to the right (west) of the sun. It passed between sun and earth (inferior conjunction) two weeks ago, and it has been separating swiftly to the sun's right since then. As it resumes its easterly motion (the same direction in which the sun moves through the stars), its rate of separation from the sun slows down until, on the 15th, it reaches its maximum distance (elongation) from the sun. From now until the end of March , Mercury will be in position to be seen as a morning star, but westerly (morning) elongations are not favorable at this time of year. The planet rises too late and too slowly to be seen during morning twilight.

March 3: You may still see the waning crescent moon this morning in the late twilight, but not on the following mornings. Next time you see the moon it will be an evening crescent, in the western sky after sundown, several days after it passes the sun on the 6th (new moon).

March 8: The moon is at perigee today, the position in its orbit where it is nearest to the earth. One effect of perigee is to increase the moon's tidal force, so that high tides are higher than normal and low tides lower. The tidal range at perigee is about 20 percent greater than the average range. The greatest tidal ranges occur when perigee occurs close to new or full moon, but new moon this month was two days ago.

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March 11-12: The bright star near the crescent moon tonight is Aldebaran, in Taurus. It appears well up in the south after sunset, with the star just below and a little to the left of the lower horn (tip) of the moon. During the evening, the moon will move to the left above the star, passing closest to it about 12 a.m. on the 12th (Eastern standard time), then moving off to the east (left). Moonset occurs at about midnight, local time. The moon passes so close to the star that it covers Aldebaran (an occultation) over northern Canada and Alaska.

March 15: Mercury is at greatest westerly elongation, its greatest distance to the sun's right (west). This places the planet in the best position to be seen as a morning star, but the chances of seeing it vary widely from one elongation to another, depending on several factors. This is not a good elongation, because Mercury's position, to the sun's right, places it farther south, where it rises too late and too slowly before the sun to be seen easily.

March 20: The sun arrives at the vernal equinox at 12:03 p.m. EST, and spring begins in the Northern Hemisphere (autumn in the Southern Hemisphere). At the vernal equinox, the sun is directly above the earth's equator, moving from south of the equatorial plane to the north. The word "equinox" means equal nights, implying that day and night are equal in duration (12 hours long each) when the sun is at this position. This is true theoretically, but not actually, because of the way we calculate the length of the day. From sunrise (when the upper edge of the sun first appears above the horizon) till sunset (when the upper edge sets), the interval today is about 12 hours, 8 minutes.

March 20-21: Planet-watchers will find Jupiter and Saturn interesting tonight , and nonplanet-watchers should find them particularly easy to locate. The moon is the reason for both. It's full moon tonight, and the moon, Jupiter, and Saturn rise almost together and move across the sky together through the night. The two planets will be the onlym objects you can see near the moon, the only two bright enough to shine through the scattered moonlight in the atmosphere (Jupiter is the brighter of the two planets). Shortly after dark, the rising moon will be above the two planets, and it may seem that all three objects are climbing through the sky at the same rate. Not so, however, as you can see by watching them carefully, even for an hour or so. It will appear that the moon is rising more slowly than the two planets. What's really happening, of course, is that the moon, moving to the left (east) around the earth, is sliding past Jupiter and Saturn, passing above Jupiter at about 8 p.m. EST, above Saturn about three hours later. After it passes them, the moon will separate slowly to their left.

March 24: The moon is at apogee, the position in its orbit where it is most distant from the earth.

March 26-27: Jupiter and Saturn have been "morning stars" until now, even though, for the last two months, they have obviously been prominent objects in the early evening sky. But until now, they have been rising after sundown, remaining in the sky after sunrise, technically placing them in the morning sky (to the sun's right, rather than to its left). On the 26th, Jupiter is at opposition, and on the 27th, Saturn, relative to the sun, which means that they are neither to the right nor left of the sun, but halfway around from it, opposite in Earth's sky. At this position, they rise at sunset and set at sunrise, remaining above the horizon all night. They also leave the morning sky and become "evening stars." From now on, they rise beforem sunset and set before the sun rises.

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