Angular measurement

[marcdearn]

OK....This might me being stupid but......

The angular "distance" between Dubhe and Alkor is around 25 degrees (so it tells me in Astronomy magazine) and the moon is half a degree

This means that 50 full moons can fit between Dubhe and Alkor????????????

Not the full moon I see !!!

Where am I going wrong?

Sorry for the silly query

[Merlin66]

The answer is correct.

The optical illusion is that the moon "appears" actually larger than it really is.

The moon looks larger when rising or setting. Our eye/ brain combination considers the sizes of nearby objects like buildings and "transfers" this information to our perception of size of the moon.

[Demonperformer]

CdC gives the separation as 19d 20', and the diameter of a full moon as 33', which would make it 35 full moons. You also need to consider that the closer to the horizon an object is, the larger it appears due to diffraction. Also, apparent separations can be deceptive. As a simple test, hold a piece of card at arms length, with one edge on one side of the moon when it is high in the sky and mark the other edge on the card. Then use the same card to measure the distance from Alcor (on the same edge you used as the moon) and mark the position of Dubhe. Compare the two lengths. I think you will be surprised.

HTH

EDIT: sorry ... merlin got in while i was typing

[brianb]

And bright objects always look bigger, a phenomenon know as irradiation - for instance, when you see a crescent moon with the "dark" side lit by earthlight, the bright crescent appears to be much bigger even though the two parts obviously have the same physical diameter.

[marcdearn]

Thanks for the answers.....I knew it was right it just didn't seem it intuitively!

1

Good idea for the card measurement....better than an outstretched fist anyway

[ollypenrice]

Hold a pencil upright at arm's length. It will easily hide the full moon. Hard to believe but try it.

Olly

[Psychobilly]

Wasnt in SPM himself who did the pebbles onthe beach experiment?