StarWatch for the greater Lehigh Valley



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1016    FEBRUARY 7, 2016:   Get Sirius Punxsutawney Phil!
After a firm knock on his door with a cane, Punxsutawney Phil waddled from his den and did not see his shadow this past February 2. In his odd way of observing nature, he proclaimed an early spring, at the very least consistent with the way winter has been progressing, despite the record snowfall of January 23. Keep in mind, the snow was gone basically two weeks later which may be a record in itself. What Phil is missing, if he’d take a brisk walk outdoors at night, is that after sundown, the winter star patterns are all ablaze in the south, a true testament to the reality that “it ain’t over astronomically until the Virgo lady sings.” Virgo with her blue supergiant star, Spica, is a true harbinger of spring and the impending warmth to come. Spica will be in the SE by 10 p.m. EDT on IRS Day, just like the best star of winter, Sirius, is in the SE around 7 p.m. right now, and due south by 9 p.m. Sirius the Dog Star, also the nose of Canis Major, the Big Dog, is only second in brightness to the sun. If you are still not sure where it is, simply follow the three belt stars of Orion downward and the “scorching” star will be there. Sirius, plainly visible from urban locales, twinkles or scintillates madly like no other luminary in the sky. The twinkling results from its low altitude and the long column of moving air through which we are observing it, and the fact that because of its distance, it appears as a point source in the sky. The air bends and refracts Sirius’ light causing it to shimmer rapidly and change color, going from blue to red in just an instance, particularly if it is a windy night. There are also two basic reasons for Sirius’ luster. The Dog Star is a brighter star than our sun, radiating over two dozen times Sol’s energy each second, but mainly it is a result of its closeness to us. Sirius is only 8.6 light years distant, but it is a world onto itself, with a companion white dwarf star in a relatively close orbit around it. Get serious about Sirius. By late March its luminescence will be gone, replaced by the relatively bland spring sky.

1017    FEBRUARY 14, 2016:   AM/PM Confusion
Time is something that we all take for granted, but it is steeped in astronomical observations. The rotation of the Earth, which cycles in 23 hours, 56 minutes, forms its basis. During that interval the Earth has revolved around the sun by about one degree, moving the sun eastward or counterclockwise by the same amount. If this trend were to continue unchecked, in one-half year the sun would be rising at 6 p.m. and setting at 6 a.m., just the opposite times of what we would consider day and night. To complete a 24-hour cycle called a day, we add four minutes to the Earth’s rotation to return the (mean) sun back to its due south position and its high point in the sky. This moment in time when the sun is due south and on the meridian creates great confusion for societies that use the a.m. and p.m. system. After the sun has passed the meridian, it is p.m. or post meridiani, Latin for after the meridian, and a.m. or ante meridiani, designating before the meridian. But what time is it when the sun is on the meridian, or 12 hours later, when the sun has reached its lowest position below the horizon? Is noon or midnight, 12 a.m. or 12 p.m.? If we used a 24-hour clock system, then noon would be 12:00 and midnight would be 00:00 (new day). Some people believe that since the moment after midnight represents the new day, then midnight should be labeled 12:00 a.m. The moment after noontime we start saying p.m., and thus noontime should be labeled 12:00 p.m. However, here is the rub. There is no agreement among English speaking countries which defines whether noon or midnight is a.m. or p.m. or whether midnight represents the old or the new day. Anyone can do precisely as she or he wants. My rule to my students, and actually what is suggested by the experts, is to label the upper meridian transit or crossing of the sun as simply noon and the nadir meridian crossing as midnight. Or better yet, let’s consider using, like most other nations, a 24-hour clock instead. Got it, maybe? Class dismissed!

1018    FEBRUARY 21, 2016:   The Day and the Year
When we contemplate the length of the day, we incorrectly consider that it is the same length as the rotation of the Earth. The amount of time it takes the Earth to spin once about its axis is 23 hours, 56 minutes. That is the amount of time it takes a fixed object, such as a star, to return to the same place in the sky. Astronomers take this period and divide it into a 24-hour interval called the sidereal day. During a sidereal day, the Earth’s revolution around the sun shifts its orbital position by approximately one degree, causing the sun to move by that amount towards the east. Because we want the sun to return to a fixed position (south) each day, we need to add four minutes to the sidereal day to bring the (mean) sun back to its same location. This gives us our 24 hour time interval that we see on our clocks and which we call the day. Thus, even though a day is based on the Earth’s rotation, the two are not the same. Enter revolution which is the orbiting of Earth around the sun. Our yearly calendars do not reflect the true orbital period of our planet which is 365.2564 days. Instead our calendrical years are based upon the sun crossing a slowly westward moving point in the sky called the vernal equinox. It is at the intersection of the projection of Earth’s equator into space (celestial equator) and the path of the sun in the sky due to Earth’s revolution (the ecliptic). The vernal equinox, where the sun crosses the celestial equator from below to above, marks the start of the tropical year, 365.2422 days in length. This point moves westward in a 26,000 year cycle called the precession (wobble) of the equinoxes, created by the sun’s gravity trying to upright Earth’s tilted axis. By focusing on two successive crossings of the vernal equinox instead of Earth’s orbit, we keep our calendar in seasonal step with all fixed dates at the expense of a slowly changing seasonal sky. In 13,000 years, the winter star patterns, like Orion, will be gracing our summer sky, and summer stars will be seen in the winter. I can’t wait!

1019    FEBRUARY 28, 2016:   A Leap for Faith
When Julius Caesar was in Egypt, he was introduced to Cleopatra’s astronomer, Sosigenes of Alexandra. Sosigenes introduced Caesar to a solar calendar which included the concept of leap years to keep the sun more in step with the actual time period it takes for the Earth to revolve around the sun. The true measure of our year reflects the time interval between two successive passages of the sun across the vernal equinox. This is called the tropical year, and it is 20 minutes, 24.5 seconds shorter than the orbital period of the Earth. Keeping a correct tropical “beat” insured that the seasons would always be in sync with the fixed dates of the calendar. Christmas would always occur near the beginning of winter, Easter in the spring, and the summer solstice in June. The Julian calendar which was a good approximation of the tropical year took effect in Rome in 45 BC and became the predominant calendar of Europe and the Americas until the gradual adoption of the Gregorian calendar introduced by the Church in 1582. The problem with the Julian calendar was that it incorporated a leap year every four years making the tropical year equal to exactly 365.25 days, when in actuality it was closer to 365.24 days. In other words, the Julian year was just over 11 minutes too long. Because the year reset on the vernal equinox, the Julian calendar gained about one day every 128 years, causing the dates of the vernal equinox to slip backwards. Future Easters would fall on earlier dates, and eventually, become out of step with Christ’s actual resurrection which occurred near the time of the Jewish feast of the Passover (spring). Also the fixed date of Christmas and the backwards sliding dates of Easter would eventually have to coincide. Enter the Gregorian calendar, spearheaded by Pope Gregory XIII (1582), which reduced the tropical year to 365.2422 days by making only century years divisible by 400 leap years. The new Gregorian calendar became accurate to one day in roughly 3300 years. February 29 is leap year day.

[February Star Map]

[February Moon Phase Calendar]