Observer's Notebook for April 1999 - Between Winter and Summer

by John Rummel

April 1999 is a month of transition. Winter has already surrendered to spring, and the days are growing longer again. April gives us one last chance to enjoy the winter constellations before they are replaced by the stars of spring and summer. Daylight savings time starts the morning of April 3, so after that, it gets dark an hour later each night (but also stays dark an hour later each morning, in case you go out to do any morning stargazing).

April evenings find Orion high in the southwestern sky. Each night the sun sets a few minutes later, and Orion drifts further and further west and lower to the horizon. This is your last chance to take a peek at the Orion nebula, just below the three belt stars. We also bid farewell to the Pleiades and Taurus's V-shaped Hyades cluster before they vanish into the sun's glare.

By April 5th, Orion, the Hyades, and the Pleiades are lined up left to right above the western horizon shortly after sunset, with Venus shining brilliantly just below the Pleiades.

Gradually gaining altitude each night, Venus passes just a few degrees to the left of the Pleiades (about the width of two fingers at arm's length) on April 11. Venus is the brightest object in our skies after the sun and the moon. Shining at a magnitude -4, it will completely dominate the evening sky throughout the spring and summer. Through binoculars it is dazzling, and in a telescope it can be positively blinding. Depending on the time of the evening, you may notice a halo of rainbow colors surrounding its tiny disk. This is the effect of Earth's atmospheric turbulence, and almost always accompanies views of Venus since we have to look at it so low to the horizon. Since Venus lies closer than Earth to the sun, it goes through phases like the moon does, but they won't be detectable for another few months. By June it will appear as a last quarter (half-moon shape) and by July you'll clearly see a crescent Venus (though a low power telescope is necessary to enjoy this view).

On the 17th, the crescent moon joins the show, passing right through the Hyades cluster on the 18th. In fact, on the evening of the 18th, the moon will occult, or cover, the bright orange star Aldebaran just before moon set. Around 9:30 on the night of the 18th, grab a pair of binoculars and watch the distance between the star and the crescent decrease. Sometime between 10:10 and 10:25 the star will suddenly wink out of existence as it passes behind the darkened edge of the moon, a wonderful event to observe.

Jupiter and Saturn have dominated planetary viewing for the last 10 months or so, but this month Mars takes center stage with its closest approach to Earth since the summer of 1997. Rising in the southeast, it should be visible above your horizon by shortly after 9:00 early in the month, by 8:30 at mid-month, and by the end of the month, it will be well up into the southeastern sky by sunset. Look for it just to the right of the full moon on April 29th.

Mars will be at it's closest point to Earth during that last week of April, but you'll need a pretty good telescope to show you anything more than an orange dot. At just half the size of Earth, it can't compete with the breathtaking views of the larger gas giants, but its color and proximity to Earth always make it an irresistible target for amateur sky-watchers.

Enjoy the transition from winter to spring, and don't miss any opportunities to get out of town and find the darkest skies possible for full enjoyment of the April sky.

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