## Halo around the full moon

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ianseeley
Asternaut
Posts: 1
Joined: Wed Mar 03, 2010 3:36 am

### Halo around the full moon

I am a complete amateur, but an avid skywatcher and APOD follower. Please excuse the following (perhaps) dumb question.

I have recently noticed a large halo around the moon, that had an apparent radius of the length of my hand at arm’s length.

Given that my hand measures 8 inches from wrist to finger tips, and my eye is 22 inches from my hand at arm’s length, how high are the clouds?

If I knew the angle of refraction, I could then estimate the cloud ceiling on a night such as tonight, where the halo is roughly one finger joint or one inch.

Probably a very simple task? (I’d probably do better using an actual ruler but the hand measurement is coarse but very convenient).
I would like to make a simple calculator that would give me this data easily on these clear winter nights.

Many thanks again to all of you who help us in our eternal quest for enlightenment,

Ian Seeley

Chris Peterson
Abominable Snowman
Posts: 15321
Joined: Wed Jan 31, 2007 11:13 pm

### Re: Halo around the full moon

ianseeley wrote:I have recently noticed a large halo around the moon, that had an apparent radius of the length of my hand at arm’s length.

Given that my hand measures 8 inches from wrist to finger tips, and my eye is 22 inches from my hand at arm’s length, how high are the clouds?
tan(theta) = 8/22, so theta = 20°

What you are seeing is the most common type of ice halo, which has a radius of 22°. However, since it is the angle that is constant, not any absolute size, there is no way to determine the cloud height. All you really know is that since this type of halo is normally produced by cirrus clouds, they are high- typically 7 km or more.
Chris

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Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

wonderboy
Commander
Posts: 569
Joined: Sun Jan 31, 2010 11:57 am
AKA: Paul
Location: Glasgow, Scotland

### Re: Halo around the full moon

This is caused by cloud suspended ice crystals refracting light from the moon. I remember I saw one of these one night and loved it. In 25 years of life, I've only ever seen 1. You also get them around lamp posts and other light sources when there are ice crystals hanging in the air. its quite remarkable but simple really.
"I'm so fast that last night I turned off the light switch in my hotel room and was in bed before the room was dark" Muhammad Ali, faster than the speed of light?

Chris Peterson
Abominable Snowman
Posts: 15321
Joined: Wed Jan 31, 2007 11:13 pm

### Re: Halo around the full moon

wonderboy wrote:This is caused by cloud suspended ice crystals refracting light from the moon. I remember I saw one of these one night and loved it. In 25 years of life, I've only ever seen 1.
You need to get outside more! Lunar and solar halos are very common. They happen many times every year whether you live at the poles or the equator, and whether the Moon is full or just a crescent.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

wonderboy
Commander
Posts: 569
Joined: Sun Jan 31, 2010 11:57 am
AKA: Paul
Location: Glasgow, Scotland

### Re: Halo around the full moon

I'll be honest, theres not one night that I don't look at the stars. I look up most of the time!
"I'm so fast that last night I turned off the light switch in my hotel room and was in bed before the room was dark" Muhammad Ali, faster than the speed of light?

owlice
Guardian of the Codes
Posts: 8376
Joined: Wed Aug 04, 2004 4:18 pm
Location: Washington, DC

### Re: Halo around the full moon

Wonderboy, check this out: http://www.atoptics.co.uk/opod.htm

A closed mouth gathers no foot.

wonderboy
Commander
Posts: 569
Joined: Sun Jan 31, 2010 11:57 am
AKA: Paul
Location: Glasgow, Scotland

### Re: Halo around the full moon

Thats pretty cool. looks like theres some lenticular clouds on the go there as well. Not as dramatic as some lenticular clouds Ive seen but I think they're ace aswell. Thanks.
"I'm so fast that last night I turned off the light switch in my hotel room and was in bed before the room was dark" Muhammad Ali, faster than the speed of light?