JUNE  1998

JUNE STAR MAP | INDEX

093    JUNE 7, 1998:   Magnitude

We are surrounded by astronomical objects of varying brightnesses, both during the day and night. Daylight sightings of the moon are easy. Even Venus can be seen in the daytime, if the sky is transparent enough and you know exactly where to look. Forty-five minutes after sunset, any of the planets that are visible to the unaided eye can be viewed, given proper placement and sky conditions. At night, hundreds of stars of different brightness intensities can be observed from suburbia. Measuring the magnitude of a star all started with the Greeks, who qualitatively noted that the brightest luminaries were of the first magnitude and the faintest stars were of the sixth magnitude. Strangely with magnitudes, the more positive the number, the fainter the object becomes. Presently, astronomers have quantitatively defined the precise brightness of the stars and planets, but have kept the Greek's more positive-fainter rule. A difference of one magnitude equals an intensity change of 2.51. This other unusual concept is made more understandable with the realization that a difference of five magnitudes equals an intensity variation of 100. So with this in mind, the sun’s magnitude is a staggering -26.7, the full moon, -12.6, and Venus about -4.4. The stars of the Big Dipper range from +1.8 for Alioth to +3.3 for Megrez. The stars of the Little Dipper vary even more, from +2.0 for the North Star to +5.0 for Eta Ursae Minoris. The faintest stars visible in the urban Lehigh Valley are probably about +4.5, but at Hawk Mountain, you can most likely reach +6.0 on a really clear night. That’s an intensity variation of nearly four, and it makes all the difference in what you’ll be able to view in the sky between an urban and rural locale.      Revised May 17, 2006

094    JUNE 14, 1998:   Summer Solstice

The summer solstice is almost upon us, but did you know that many similarly related events involving the Earth and the sun are also happening near the time of the solstice? Take for instance, that today marks the day of earliest sunrise for our latitude. For Allentown specifically, the sun rose this morning at 5:30 a.m. On the 14th, the sun will be above the horizon for 15 hours, 3 minutes. Then on the 21st, at 10:03 a.m. EDT, the sun reaches its highest position north of the Celestial Equator, summer solstice, the beginning of a new season and the longest day of the year. Sunrise to sunset in Allentown takes 15 hours and 5 minutes. Even though the summer solstice marks the longest day of the year, the sun continues to set later each evening until June 27th. However, the sun also rises later too, and this offsets the later sunsets so that the sun will be in the sky for 15 hours, 4 minutes on the 27th. Allentown’s sundown on June 27th is at 8:36 p.m. Finally, the Earth revolves around the sun in an elliptical (oval-shaped) orbit arriving this year at its greatest distance from the sun, aphelion, on July 3rd at 8:00 p.m. The sun will be almost exactly 94.5 million miles away from us at that time. If you are thinking there must be some mistake, you’re wrong. The seasons have nothing to do with Earth’s changing distance from the sun, but are caused by its axial tilt which is 23.5 degrees from the perpendicular of Earth’s orbital plane.

095    JUNE 21, 1998:   Polaris and Your Latitude

One of the two coordinates that allows you to find your position on the Earth’s surface is latitude, the angular measure north or south of the equator. Polaris, the North Star, can easily allow you to observe your change in latitude if your travel plans take you in a northerly or southerly direction. The altitude of Polaris approximates your latitude position to within a degree. The Pointer Stars of the Big Dipper will allow you to trace a path to Polaris. For Allentown, the North Star’s elevation is approximately 40 degrees. That’s equivalent to four clenched fists held at arm’s length (thumb on top), stacked one on top of each other. Let’s say that you’re traveling to Disney World this summer. You’re out by the pool after dark, and the kids are swimming away their last bit of energy before bed. Check out Polaris; you’ll find it about three fists high, 28 degrees above the horizon. Traveling to southern Europe? The chance will be negligible. Rome (42 degrees N) and Athens (38 degrees N) are too close to our latitude to note any changes by using your fist. However, London is about 52 degrees N, five fists high for Polaris, and southern Alaska, around Anchorage, is about 61 degrees N. That’s six fists, but measurements are impossible until later in the summer. Guess why? Finally, if Hawaii or a Caribbean cruise is in the offering, Polaris will hug the horizon at two fists or less.

096    JUNE 28, 1998:   Mercury Lurking

The planets again begin slowly to take center stage. They have been missing for so long from our evening sky. For the next three weeks Valley observers will have the opportunity to catch illusive Mercury which is usually hidden in the bright glare of the sun. This was the planet which a dying Copernicus in 1543 lamented he had never seen. Copernicus favored a sun-center hypothesis in his description of the universe, and he reawakened Europe to its importance. Mercury will still not be an easy target, but it is worth trying to observe. Over the past several weeks its orbital motion has carried it to the east of the sun which means that Mercury will be visible after sunset if you have an excellent western horizon and very transparent sky conditions. Presently, Mercury is just to the left of Caster and Pollux, the Gemini Twins, slightly less than 10 degrees above the horizon at 9:00 p.m. Use binoculars. By July 17, Mercury will have reached its greatest eastern elongation from the sun, nearly 27 degrees, but it will still only be about 8 degrees above the horizon at 9:00 p.m. After this date Mercury rapidly begins to come around towards us in its orbit, quickly moving downward and into invisibility for Lehigh Valley observers. Look east near the horizon for Venus which will be playing tag above the star Aldebaran in the dawn sky this week (5:00 a.m.). They are closest on Thursday. We reach the midpoint of the year on Tuesday.