Perpetual Chinese Calendar

LunarCal  User's Guide - Features

Moon Compass

This screen is used as an aid to visual observation of the Moon. On the left of the screen is an altitude semi-circle representing the vertical position of the Moon. The horizon is denoted by 'Horz'. If the Moon (marked by a red crescent) is below the 'Horz', then the Moon cannot be seen at the particular instant in time from the selected city.

To the right of the semi-circle is the azimuth circle, and it represents the position of the Moon in relation to the compass bearings.

So with the Moon's altitude and azimuth, you can locate the Moon in the sky.

The current Moon's position is shown graphically above. The semi-circle shows the Moon's altitude above and below the horizon and the full circle shows the position of the Moon with respect to the compass bearings North-South-East-West (N-S-E-W). 

The altitude is measured positive +ve from the horizon upwards whereas the azimuth is measured from the compass bearing North direction clockwise. The altitude is measured referenced against the 0 horizon. This value differs from the NASA's value (from NASA's web site) by an amount equal to the equatorial horizonal parallax term at that instant. I choose to use absolute altitude. The difference is insignificant for general use.

The above shows the Moon to be 13.66 degrees below the horizon and 251.81 degrees measured from North clockwise. The display uses short form 'Alt' for altitude and 'Azi' for azimuth.

Julian Day, UTC and local time are explained in the terminology section of this user's manual.

The Moon age is the fraction of days since the New Moon date. The "Frac Illuminated" is the percentage of the Moon surface (as a circular disk) that is illuminated by the Sun and appears as the bright portion of the Moon.

The picture of the Moon is exactly as you will see the Moon at the instant of time indicated at the selected location. By using the [UpArrow] [DnArrow] [LeftArrow] and [RightArrow] keys you advance/decrement the clock to observe the Moon's orbital position and phase.

The moonrise and moonset is the 1^st occurring moonrise/set. In higher latitudes (greater than 40 degrees North or South), there may be more than one moonrise / moonset. Only one such set of data  is shown.

Precession

The above graphical picture of the moon shows the phase exactly as you will see it. However, precession - the conical rotation of the Moon's North Point is not yet implemented. The graphics manipulation is fairly extensive and a 3D picture of the Moon is required. This value too small to be shown with sufficient accuracy. Perhaps someday in the future when I get my CCD camera and telescope I may implement it. But this is purely an academic exercise.

Moonrise & Moonset

Sometimes the program says that there is no moonrise but you can see the Moon. Why is this so? This is because the Moon had already risen the day before - say at 23:55pm yesterday, but it has not yet set so we can still see the Moon even though there is no moonrise today (tonight)! Every moonrise is followed by a moonset, but not necessary within the same day. When the program says that there is no moonrise / moonset we mean that the event did not happen on the day.

The Moon has Flipped!

The bright portion of the Moon appears to have shifted after the Moon had passed overhead! Has LunarCal gone bonkers? No not really!

If you care enough to spend some time visually observing the Moon carefully, you will notice the Moon appears to suddenly flip over - the bright and dark (shadow) portions of the Moon seems to have changed places after the Moon passes overhead. This is an interesting event many people never seem to notice or realize.

When the Moon crosses a point (the observer's Zenith as indicated by the letter Z on the compass screen) which is directly overhead (draw an imaginary straight line from your feet through your head and upwards to the sky), the bright and dark portions swaps. The Moon actually did not flipped! Its only an illusion. Initially, during the Moon's rising stage, you are looking approximately at an easterly direction, then when the Moon crosses overhead and begins its descend, you would be looking approximately at an westerly direction at the Moon.

Experiment

You can simulate the Moon flip over phenomenon using your hands. Just stretch both hands outwards - left & right sides of your body like a T. Now look at you left hand. Turn the left palm facing you slightly instead of upwards. Your fingers are now pointing upwards. Look at your left hand thumb position - it is on the left side of your palm, yes? Now swing you left hand slowly upwards (keeping the arm straight), then over your head and try to reach the right palm with your left. Your left palm's fingers are now pointing downwards and notice that the thumb of your left hand has now flipped over to the right side!

In the US Naval Office's (USNO) website, you can also see the what the moon looks like at the current time (USNO calls it Apparent Disk of the Moon). But that picture you see will not be the same as LunarCal's image. This is because USNO shows what the Moon looks like from the center of the Earth looking directly at the Moon - so there is no "overhead" position, thus there is no flipping of the Moon.