Dome rebuilt

Here is the sky for Austin, Texas, during the April 8 eclipse.

See the end note about enlarging illustrations. The Moon is exaggerated 5 times in size.

We can call a picture of this kind a dome view, as distinct from a horizon scene like this for the dawn of the same day:

Both use altazimuth projection: the position of every point is calculated from its altitude (height above the horizon) and azimuth (left-right distance parallel to the horizon) and translating it into angular direction and distance from some central point. If that focus point were on the horizon, the horizon would be a straight line. In the horizon scene, I’ve chosen a focus in the south-east and 10° below the horizon, which therefore appears as a convex curve. In the dome view, the focus is the zenith, so that the horizon becomes a circle. In this way the whole sky can be shown.

The Astronomical Calendar has always used an evening dome view for each month. The “Eclipses” section of Astronomical Calendar 2024 includes dome views for the slight lunar eclipses of March 25 and September 18, but not for the major solar eclipse that’s coming on April 8.

The purpose of such a picture for an eclipse is mainly to alert you to where to look for stars and planets that could become visible as the sky darkens, when the glare of full moonlight is dulled by Earth’s shadow, or, more suddenly and deeply, when the blinding Sun is stamped out by the Moon.

A dome view picture needs a lot of space, and I failed to find a way to fit a third one in.

A few days ago, Jan Brustmann in Ohio sent me a question about features in the sky surrounding the eclipse. I sometimes feel a bit of impatience – it’s going to consume time, I’m not an answer bureau. But in hastily searching for an answer, I realized that it would have been useful to have made a dome view. So we have Jan to thank that I’ve belatedly made one.

I had to revive a programming scheme used years earlier; as usual, that was affected by changes I had made in the meanwhile, so that it was riddled with errors. It took me more than a day to root them out.

And then to take advantage of the opportunity of improving: make the picture more simply clear, and closer to being instantly ready, with less need for adjusting details.

And then we can roll out similar pictures for other locations. Here for comparison is the sky at Montreal when total eclipse reaches it about an hour later. The Moon-Sun pair is lower toward the western horizon. A southern slice of the sky, inhabited by the star Achernar, is below the horizon.

Here is how on April 8 the planets are arrayed from left to right in order of their elongation (angular distance from the Sun), with their current brightness (in the astronomical system of magnitude, in which “1” is brighter than “2”).

Jupiter      30° east      -2.0
Mercury       6° east       4.3
Venus        15° west      -3.9
Saturn      35° west            1.2
Mars        36° west            1.2

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

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” or ”Open image in new tab”, 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. Or you can click ‘Refresh’ to get the latest version.