Astro and the Moon

[Neil]

At the risk of my exposure as a complete duffer, I'll pose this question to the cognoscenti. At the sight of a near full moon in a clear sky, I thought I'd dust off the sextant and re-familiarise myself with moon sights. Using an artificial horizon, I took the sight, noted the time and cooked dinner. Afterwards, I reduced the sight and came up with the wrong answer. I couldn't find my mistake, so thought to take another sight. The apparent altitude (at around 10.00 pm) was 58 degrees odd. I'd got to the moon correction tables only to find that for the first correction, the tables only went to 34 degrees of apparent altitude?

What am I missing here? or is it just too late at night? (for me, not the moon!)

I have to admit that the only couple of times I did reduce a moon sight, it was with Cunliffe's book constantly at hand - it's not something I've practiced a lot.

 

[AHoy2]

When using an artificial horizon you have to halve the measured altitude.

 

[Neil]

Yeah, the measured altitude was 117o 16.6' But I've a feeling that it's something equally obvious!

 

[BobPrell]

I just had a look in Norie's Nautical Tables.

All the tables went to 88deg. What tables are you using?

If a set of tables was to be made smaller, maybe it would only go to some lesser angle, because the sea horizon might not be visible when the moon is way above the horizon. This is just a guess.

 

[timbartlett]

Yeah, the measured altitude was 117o 16.6' But I've a feeling that it's something equally obvious!

Forgive me if I'm stating the excessively obvious, but the Moon Altitude Correction table is in two parts -- the first part deals with altitudes from 0-35, and the second part with altitudes 35-90

 

[BobPrell]

BTW Neil, Your sextant must have been at its limit measuring that angle.

did that make it any harder to do, in some way we havn't thought of?

 

[Neil]

Yes, exposed as a fool. Bright-eyed and bushy-tailed this morning and the table is in two parts, on different pages. Why, I wonder.......? It was the intervening text that threw me. Never had occasion to go over 34 before, the couple of times I've done the moon.

Most sextants go up to 120 degrees of arc, as I understand it, so 117 was near its limit.