Two Queries from a beginner

[Mike21 (Michael)]

Hi All,

I've been bashing away at this hobby for six months now. Most queries have been solved either by my prior understanding of geometry or by web research. Try as I have though, I cannot solve two questions and I've joined IceInSpace in the hope that one or several of you boffins can help.

1. How do I calculate local Midday/Zenith time? Is it midway between times for sunrise and sunset? If not where do I find a chart that tells me solar zenith times for Newcastle? I want to use the Midday shadow to mark a perfectly accurate North/South line across my deck. Every compass I've used so far only points North-ish, depending on its mood.

2. Why does the Moon "flutter"? I cannot work out why the Moon has a different elliptic across the sky every night. Doesn't the Moon orbit at right angles to the Earth's axis?

[mental4astro (Alexander)]

Hi Mike,

Question 1, I'd better leave to someone else (a man's gotta know his limitations!).

Question 2, the moon's plane of orbit is not on the same plane as earth's, there is a slight tilt, how much I'm don't know.

Combined with earth's axial tilt, the moon's flutter is a seasonal variation, which can also be seen in it's varing tidal influence during the year.

Hope that's a start.

Mental

[Mike21 (Michael)]

Thanks Mental4asto, you've jolted something in my head. I remember now from school, that it varies from Earth's plane by about 5 degrees. Ta.

[dannat (Daniel)]

here is a link for solar noon http://www.srrb.noaa.gov/highlights/sunrise/gen.html

[Mike21 (Michael)]

Brilliant, thanks Daniel.

[Blue Skies (Jacquie)]

Originally Posted by Mike21

1. How do I calculate local Midday/Zenith time? Is it midway between times for sunrise and sunset? If not where do I find a chart that tells me solar zenith times for Newcastle? I want to use the Midday shadow to mark a perfectly accurate North/South line across my deck. Every compass I've used so far only points North-ish, depending on its mood.

I see someone has already sent you a link to a nifty calculator but others may like to know that you need to have a look what is known as the Equation of Time to work out your local noon. As the Earth's orbit is elliptical, not perfectly circular, it means that at times of the year the earth speeds up a little as it heads into perihelion (around Jan 2nd or so) and then slows down as it heads out to aphelion. The graph for the Equ. of Time shows this slowing and speeding up and when local noon is compared to clock noon.

2. Why does the Moon "flutter"? I cannot work out why the Moon has a different elliptic across the sky every night. Doesn't the Moon orbit at right angles to the Earth's axis?

The moon's orbit is tilted about 5 degrees to the Earth's equator. But this isn't all - the nodes, the point where its orbit crosses from north to south and south to north, regress a little each time the moon completes one orbit. Eventually they slip around enough to line up with sun again, and we get eclipses - this is why we only see eclipses every 5.5 months or so and not any other time. The ancients, such as the Babylonians and the Chinese, knew this pattern repeated every 18 years 11 days and 8 hours and used 54 years worth of observations (3 saros cycles) of every possible pattern to predict the possibility of an eclipse with good results - having a written culture over such a long time span is an advantage! Getting a book on eclipses will probably explain it better at length than I can, but it is fascinating subject.

[Mike21 (Michael)]

Crickey, looks like I still have much to learn. The regression of the nodes and relevance to eclipses has sewn a few things together.

Thanks to you too Jacquie.

[Blue Skies (Jacquie)]

The learning is never ending, just enjoy the journey and don't worry so much about the destination. No one can know it all!