More on the Moon Graph

What's it for?

The moon shadow adjustment graph is intended to be an object of curiosity, for inspiring interest in the relationship between the movement of the sun and moon across the sky. It is of little value if you want to know the precise time as there are too many error factors involved. You can, however, get the time within half an hour's accuracy.

It was included on the dial because there are few locations where it seems more appropriate to have one than  where the sun is absent for three and a half months per year and where the moon stays above the horizon for nine days at a time around full moon time. If you don't have a watch on you, you can be completely unaware of the time up here when it is dark all day.  A glow in the southern sky around the middle hours of the day can be your only indication of the time.  

Only seven days before and after full moon are included on this dial, as this is the only time the moon is really bright enough to cast a good shadow on the dial.

What's the relationship between lunar and solar 'time'?

At full moon, the shadow cast on the dial from the moon's light would be 12 hours behind solar time. This is because at the point of full moon, the sun will be opposite the moon in the sky, allowing the full surface of the moon facing the earth to be illuminated. 

After full moon, the moon runs 'slow' by approximately 48 minutes per day, so you need to add on an extra 48 minutes per day past full moon, plus the original 12 hours,  in order to find solar time.  Before full moon, however, the moon runs 'fast' by the same amount, so you need to take off an extra 48 minutes per day before full moon.

A simple hypothetical example:

This example doesn't take into account half and quarter days, which would make your calculation far more accurate.  For example, if you read the dial at 09.00 clock time on Jan 15, it would not be a full day after full moon, but 3/4 of a day.  Each quarter of a day is equivalent to 12 minutes (48 divided by 4), so you would need to add 36 minutes instead of 48, plus the original 12 hours.  This is why there is a continuous line on the moon chart, to show that you need to calculate the amount of minutes according to the amount of hours which have passed in the day.   

The general opinion is that you also need to take account of the Equation of Time graph on the right of the dial, as this shows the difference from clock (or local apparent) time due to the elliptical orbit of the earth and the tilt of its axis.   

How do you read the graph?

You need to have a fairly good idea of how many days you are before/after full moon and sometimes this is difficult to ascertain just by looking at the moon. There are three diagrams at the base of the graph to show the shape of the moon on various days before/after full.  In the Northern Hemisphere the first illumination of the moon after new moon appears on the right side and spreads to the left of the moon. After full moon, the first part of the moon in shadow also appears on the right side and spreads to the left of the moon. For a brilliant animation of this, go to: http://www.bedford.k12.ny.us/flhs/science/moonphaseanimation.html  In the Southern Hemisphere, Tasmania for example, the opposite happens and the NH waxing moon is the part of the moon the SH sees when it is waning (after full moon) and vice versa. 

It is better if you know the date and time of the full moon precisely (this information is often on calendars - it is also on the chart above). The moon is only truly full for a little while, a matter of minutes, when its position is exactly opposite the sun. It does not stay full for a whole day, as many people assume. Therefore, if the moon was half a day past full for example, you would need to add 24 minutes (half of 48 minutes) to the dial reading in order to get solar time.  This is why the figures are on a graph with a continuous line rather than a table with discreet numbers as on the Queens College moon dial in Cambridge (the most famous 'moon dial', created in the 17th Century).

To simplify the reading of the moon adjustment chart, here are the steps you need to take:

Step One:

Find the time of the last/next full moon, either roughly from looking at the shape of it and comparing this to the pictures on the graph or by looking it up on a calendar/Internet/astronomy program such as Starry Night (or by checking them on the chart above - too easy!)

Step Two:

Work out how many days (and half or quarter days if you want to be more accurate) it is before/after full moon.

Step Three:

Find that number of days on the bottom axis of the graph, then find the corresponding number of hours and minutes on the line above. Add extra minutes for half days (24 minutes) and quarter days (12 minutes) if you want to get a more correct final figure.

Step Four:

Find where the shadow from the moon is cast on the dial (if there is interference from street lights, shield their glow from the part of the dial where the shadow from the moonlight would be) then subtract (if before the full moon) or add (if after the full moon) those hours and minutes. The simplest way to do this is to count around the hour markers on the dial as calculating larger figures such as these in your head in sub-zero temperatures might be a bit of a trial! Finally, if you want to be even more accurate, add or subtract minutes according to the Equation of Time graph on the Eastern side of the dial.