Crux, the Coma Hour, and school exams

We’ve had discussion about how far south you have to be to see the small, brilliant, and charismatic constellation of the Southern Cross.

A star can get above your horizon if you are less than 90° north of its declination.  The declination of Alpha Crucis (also called Acrux), the southernmost and brightest of the four stars that form the cross, is -63°.  So 27° is the latitude at which Alpha Crucis kisses the horizon, for an instant, before sinking again below it.  You would have to be a few degrees farther south if you are to discern the star.

Around the Earth on latitude roughly 30° or 35° are Los Angeles, Phoenix, Houston, Memphis, Atlanta, Casablanca, Malta, Crete, Cyprus, Baghdad, Tehran, Kabul, Islamabad, Lhasa, Shanghai.

Here’s a chart of the sky as seen from that limiting latitude 27°.

See the end note about enlarging illustrations.

Peeping above the south point is Crux, with as many bright stars as there are letters in its name.  I could instead draw it with a north-south line and an east-west line, but it looks to me less like a cross than a diamond, and that’s how it seemed to me from Uganda.

Actually Acrux is just below the horizon, because its declination is -63° and a bit.  It shows in my picture only because of the size of my symbol for it.

The stars are sliding rightward across the sky, as Earth turns leftward.  The curve of Acrux’s travel comes tangent to this horizon and then dips away below it.

As has been said, seeing the Southern Cross is a holy grail for north-hemisphere observers.  That made me remember and re-find these evocative words from Richard Hinckley Allen’s old book on star names:  “It was last seen on the horizon of Jerusalem — 31°46’45” — about the time that Christ was crucified. But 3000 years previously all its stars were 7° above the horizon of the savages along the shores of the Baltic Sea, in latitude 52°30′.”  He was referring to the age-long change of stars’ positions caused by precession.

The time chosen for the picture has to be the time when Crux is highest.  We are  talking about sidereal time: time in relation to the starry universe around us.  We need sidereal time about 12 hours; that means, when right ascension 12h is on or near the meridian.

It is what I called, in the cover picture story for my Astronomical Calendar 1976, the Coma Hour.  Crossing the meridian at this time, much farther north, is Coma Berenices with its tress of stars and galaxies.

This sidereal time – this star hour, this whole state of the sky – arrives a little earlier each day.  It comes at around midnight in March, 10 PM in April, 8 PM in May, then in daylight; then around dawn in November, 6 AM in December, 4 AM in January, 2 AM in February.  Now begins the season for quest of this grail!

And what are those stripes across my sky map?  They represent the star hours.  Each is a band between two of the main lines of right ascension.  The one now highest and in the middle, that is, crossing the meridian, is the band between the right ascension lines of 12 and 13 hours.  This band contains the middle of the Big Dipper, and Coma nearly overhead, and Corvus, and Crux.  (The 12h line runs through the autumn equinox point, where the celestial equator crosses the ecliptic.)

I call these bands “gores”; they are like the gores of textile from which a skirt or an umbrella in made.  I want to say more about, and ask for your opinion about, these star hour gores, tomorrow (I hope).

Home planet department

So, to see the Southern Cross, go down to the Caribbean, or the Mediterranean – their northern shores, or, better, their southern.  “The grand object of travelling is to see the shores of the Mediterranean,” said Samuel Johnson, according to his biographer Boswell.

We’re at latitude 52, but we just visited the Mediterranean.  That is, we spent another sunny morning in Kew Gardens, and made our way across its expanses – now daubed with blue drifts of spring crocuses – to the Mediterranean garden, which we had failed to find last time.

This photo was taken by a small Mediterranean hedgehog.

It’s like a grassy glade, surrounded by several hillocks, each of which is clothed with Mediterranean vegetation such as cypresses and cork oaks, little forests through which wind sandy paths, and overlooking is a small Greek temple (which is actually a monument to King William IV and contains pictures of British military victories).  As in all parts of the great gardens, the trees and many lower plants bear taxonomic labels, and larger signs earnestly educate the strolling public, such as school groups, about botany and conservation.

The Covid plague hasn’t stopped school open-air outings, but it has prevented exams from taking place in the normal way, so the British government has decided on a policy by which teachers will assign grades.  It makes you wonder how objective that can be.

It also caused me to wonder about the whole worldwide system of school examinations.  They seem necessary in order to frighten pupils into learning, and they divide them into layered classes for the rest of their lives.  Will there ever be a state in which the young learn as they wish, judge themselves, and are able to enter the occupations they wish?  It may be compatible only with a Montessori-like system in which children are encouraged into teaching themselves.

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ILLUSTRATIONS in these posts are made with precision but have to be inserted in another format.  You may be able to enlarge them on your monitor.  One way: right-click, and choose “View image”, then enlarge.  Or choose “Copy image”, then put it on your desktop, then open it.  On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers).  Other methods have been suggested, such as dragging the image to the desktop and opening it in other ways.

Sometimes I make improvements or corrections to a post after publishing  it.  If you click on the title, rather than on ‘Read more’, I think you are sure to see the latest version.

This weblog maintains its right to be about astronomy or anything under the sun.