Solar Halos, explanation? (APOD 16 May 2008)

[apodman]

Nice picture.

This reminds me that astronomy isn't just for us creatures of the night.

It occurs to me that this modifies my last post (a rant on light pollution) in that it's less of a problem in the daytime.

[apodman]

Once upon a time, I went on a daytime observation binge. I had planets, the waning moon, and sunspots to keep me looking. But a word about pointing optics at the sun:

I had a little catadioptric reflector that came with a solar filter to attach to the eyepiece. It worked fine, but I learned later that you're supposed to put your solar filter (and/or aperture reduction cover) on the light-gathering end of the telescope. This way you don't put all that sunlight (and the heat that goes with it) through all of your optics.

In other words, don't cook your nice camera shooting the sun. Use a proper sun filter. Same for your eyes.

[NoelC]

When I was a teenager (too many decades ago) I had a small 2-1/2" refractor telescope that came with a "Sun Filter" that screwed onto the eyepiece. I used it to look at the sun sometimes. I never thought about it much; I was doing exactly what the telescope book said to do.

One day - thankfully at a time when I was not looking through the telescope - the filter broke into two pieces, falling out of its little screw-on mount and into the diagonal. It was because the concentration of sunlight had heated the filter glass so much that the thermal stress was simply too much for it.

I had been looking at the sun just a moment prior.

What impressed me most at the time, and stuck with me since, was that if I had been looking at the sun at that very moment, quite likely the concentrated sunlight would have literally burned out my eye, even before I could blink. I would have lost my sight in a most agonizing way. I have a crystalline memory of the brilliant, hot light coming up out of the eyepiece just after the filter broke, accompanied by just a wisp of smoke, before I moved the telescope to point away from the sun.

Never, ever, NEVER EVER point a telescope at the sun without the light being properly filtered at the objective end.

-Noel

[Phil G]

The text speaks of a circle having a radius of x# degrees. I find this very confusing. In my career in land surveying, degrees were units of angular measurement, and the radius was a linear measurement. Would someone kindly explain how a radius can be measured in degrees?
Peace,
Phil G

[Pete]

Hi Phil,

The text (implicitly) refers to the angular radius of the circle. If you point an imaginary line at the Sun and a second line at the halo's rim, they'll intersect at an angle of 22 degrees. In other words, the halo rim is 22 degrees from the Sun. Along the same lines (HEY-OHH), the horizon is 180 degrees wide.

Cheers!

[Phil G]

Pete,
Thanks. Now it makes sense.
Phil G

[astrolabe]

Hello NoelC,

An important cautionary warning that can never be said TOO FREQUENTLY!Last winter a local amature astronomy club of experienced members set up a tracking telescope outside of L.L. Beans in Freeport, Me. for anyone who wanted to take the time to look at Sol. I was leary because of knowing about the potential risks but they knew the drill and I got to see my first sun spot in real time! I, needless to say, was blown away and duly impressed and thanked them for providing such a neat and safe setup. Just another day in the life.

[Sputnick]

Sun dogs and halos, among their other functions and attributes, act as weather indicators telling of a big change to come very shortly. I seem to remember both colder, wetter weather, and dry warmer weather following. Perhaps I'm confused about the effects though .. but they do signify major change.

[astrolabe]

Hello Sputnick

The high thin cirrus clouds contain ice crystals...no brainer there, but the direction and speed are the factors to watch for. Because of the altitude they may not appear to be moving very fast but if one sees them in motion then they going 50+ mph or greater, 120 is not uncommon.

When over taking a warm air mass at the surface, which is rising, preciptation will most likely occur as the cooler, heavier air mass drops to replace the warmer rising one. If the cirrus is really humming along then it allows the rising air a place to convect and vent aloft, then one can really be in for a strong event.

[Sputnick]

Astro - how's the weather in Old Orchard Beach? Sounds like a beautiful place.

[astrolabe]

Hello Sputnick,

Ahh yes, but then most places are on this amazing floating oasis we live on. But then what's the point? Why are we here? To be joyous of course! I'm feeling unusually benevolent right now so my best to you and the Forum.

[Sputnick]

Astro - "unusually benevolent" (?) From what I know of you you are normally unusually benevolent.

However .. we cannot always be joyous. Some experiences here definitely cause geat pain despite our intellectual certainty that all things work for good .. and we must respond honestly to that pain or be torn apart like two planets colliding at high velocity.

But yes .. I'm so happy that you're joyous.

[kestrel374]

"Surrounding the zenith (the point directly above the observer) and always at the same altitude as the Sun is a lovely ...."

How can the halo be at the same "altitude" as the sun?

[Qev]

"Surrounding the zenith (the point directly above the observer) and always at the same altitude as the Sun is a lovely ...."

How can the halo be at the same "altitude" as the sun?

It's a bit of sloppy language, really. The parahelic arc always appears on the sky at the same 'height' from the horizon as the Sun. It draws a circle around the sky, parallel with the horizon, that passes through the Sun.

[apodman]

If you are not blessed with an equatorial mount for your telescope, perhaps you have an altazimuth (altitude & azimuth) mount. The telescope moves horizontally 360 degrees in azimuth, along the horizon if you get it level. The telescope moves 90 degrees in altitude, from the horizon up to the zenith. Any point in the sky can be expressed as degrees of azimuth and altitude. This use of the word "altitude" is the same as for the position of the solar halo.

If you are aiming large artillery with cranks and gears, the altazimuth coordinate system is for you. If you want your telescope to follow an object while your camera captures a time exposure, you need equatorial tracking.

[Chris Peterson]

It's a bit of sloppy language, really. The parahelic arc always appears on the sky at the same 'height' from the horizon as the Sun.

"Altitude" is precisely the correct word, no sloppiness at all. However, "height from the horizon" is a little sloppy <g>.

[kestrel374]

ah! I get it. Thanks!

[Qev]

It's a bit of sloppy language, really. The parahelic arc always appears on the sky at the same 'height' from the horizon as the Sun.

"Altitude" is precisely the correct word, no sloppiness at all. However, "height from the horizon" is a little sloppy <g>.

You're right, I stand corrected!