Astro Bob blog: 'Tis brillig and the slithy toves welcome springThe new season is no nonsense. Plus watch the moon glide near the Seven Sisters star cluster tonight. And a final update on the Duluth fireball.
By: Bob King, Duluth News Tribune
'Tis brillig and the slithy toves welcome spring
My play on words comes from Lewis Carroll's nonsense poem "Jabberwocky", one of my favorites. Its first line about the slithy toves seems to hint of a brand new day ... at least to my ear.
Spring rolls in today starting at 12:32 p.m. Central time. While the sun hovers directly over the equator, poised between the extremes of its yearly path, the casual crescent moon ambles across the southern reaches of the Seven Sisters star cluster during the early evening hours.
Look high in the west-southwest during twilight to find the moon. Just a moon-diameter to its upper right you might spot a few of the Pleiades or Seven Sisters. Binoculars will really enhance the view showing dozens of stars and the dim earthlit portion of the moon contrasting sharply with the sunny crescent.
Although the moon passes south of the cluster's brightest stars, it will cover or occult a moderately bright one (magnitude 5.4) around 8:24 p.m. for the Duluth, Minn. region. Any telescope will show the moon slowly creep up on the star before it covers it in a sudden SNAP! The faintness of the earthlit portion of the moon will make this occultation an easy one to see. When occultations happen along the moon's bright, sunlit edge, glare can make the star difficult to follow.
The moon will track to the left and above the cluster as the night wears on. These panels show the moon's positions at 9 and 10 p.m. Credit: created with Chris Marriott's SkyMap program.
If you live considerably south of Duluth, the moon's path will take it further into the Pleiades; if north, it will miss the cluster altogether. Not only is it fun to watch an occultation of a star but it's instructive to track the moon's eastward movement as its orbits the Earth. Being close to so many stars gives us an easy point of reference during the night.
We face the sun squarely from the side during spring and fall so both northern and southern hemispheres receive equal hours of light. In the winter (right), the northern hemisphere is angled away from the sun. This lowers its path in the sky and makes for shorter daylight hours. In summer, the north is tipped toward the sun so it's higher in the sky and daylight is longer than 12 hours. Illustration: Tao'lunga
Equinox comes from the Latin for "equal night" because day and night around the planet are approximately equally long. We have two equinoxes a year -- vernal and autumnal. Both are times when the Earth's axis is neither tipped toward nor away from the sun but rather sidelong so both hemispheres receive nearly equal amounts of daylight. As discussed yesterday, the sun rises due east and sets due west today wherever you happen to live except at the poles.
These diagrams show the sun's path across the sky today as seen from the equator and from the northern U.S. Notice that the sun rises and sets in the same directions but its height above the horizon varies because of the difference in latitude. All equinox illustrations: Tao'olunga
At the equinoxes, the sun crosses an imaginary circle in the sky called the celestial equator. This is the sky's version of the Earth's equator. The celestial equator is concentric with the planet's equator but up in the sky so if you live in say, Quito, Ecuador, the celestial equator is directly overhead. That's why the sun is directly overhead at noon today for equatorial residents. For those of us living in mid-northern latitudes the sun will be about midway between the overhead point and horizon. Travel all to the way up to the north pole and the celestial equator is flat with the horizon (see below). Sitting on your ice floe, you'd see the sun just clear the horizon today and remain visible for 24 hours straight. Because of their unique location at the very ends of the Earth, only at or very near the poles does daylight last 24 hours long at the equinoxes. All other places on Earth have approximately 12 hours of daylight (sun visible) and 12 hours of night.
This day is an auspicious one for the poles for another reason. Starting today at the north pole the sun won't set for the next six months. Just the opposite situations prevails at the south pole is just the opposite. After today the sun drops below the horizon for six months. The tip of the Earth's axis produces a variety of fascinating seasonal effects some of which might surprise us who live in the mid-latitudes.
One last thing. Several of you have asked whatever happened to the widely observed Duluth fireball from a few weeks back. Fellow meteorite hunter Mike Bandli found two signals on Doppler weather radars that would indicate a possible fall location in northern Iowa near the town of Osage. Bandli contacted the newspaper there about any reports but nothing surfaced so the trail appears to have gone cold. Anytime you see a brilliant meteor / fireball feel free to contact me at firstname.lastname@example.org and also fill out the online report form provided by the American Meteor Society fireball monitoring program. The more sightings made and reported, the better the chances are of a potential recovery.