Emerald Selene

A simulation of a mechanical astronomical watch showing lots about the Moon

The WearOS watch face Emerald Selene is a simulation of an analog mechanical astronomical watch showing lots of information about the Moon.

The main black "moon phase" hand indicates the difference between the Moon's ecliptic longitude and the Sun's; this is a representation of the Moon's cycle through the phases. Straight up is new moon (0 age), at right is first quarter (about 7.5 days old), straight down is full moon (about 15 days), left is 3rd (last) quarter (about 22 days old). A complete lunar cycle is about 29.5 days.

Underneath the black hand is a white "Elongation" hand showing the angular separation between the Sun and the Moon in the sky. Note that for most of the time as the Moon increases from New to Full (the right half of the dial), the elongation is very close to the value of “Delta Ecliptic Longitude” (or moon phase) shown by the black hand, and the white elongation hand will be “hiding behind” the black hand. But when the elongation is close to 0 or 180 degrees, and the Moon’s ecliptic latitude is not close to zero, the elongation will be different enough for the hand to “peek out”. And during the waning cycle from Full to New (the left side of the dial), the two hands are on opposite sides of the dial and will both be clearly visible.

When the angular separation is less than about 90 degrees, we calculate it using the positions of the Sun and the Moon as seen from the observer’s position (topocentric coordinates), as that is most useful when the objects are close together. When the angular separation is larger than that, we switch to using the positions as seen from the center of the Earth (geocentric coordinates), as that is most useful close to a full moon where there is the possibility of a lunar eclipse (the shadow thrown by the Earth is centered at the Earth center, and doesn’t depend on the observer’s position).

Each dark or light section in the outermost dial represents a 24-hour day, so you can easily count how many days until the next quarter by counting sections from the main black hand to the quarter's indicator. You can also estimate the approximate time of day that quarter will occur by seeing where in its colored section it lies.

The numbers in the quarter icons show the date when that phase occurred or will occur. The two clockwise from the main hand are in the future, and the two behind the main hand show dates in the past. The main black hand passes over the indicator on the date shown.

The small dial at 2 o'clock shows the Moon's ecliptic latitude (which is its angular difference from the Sun in the dimension perpendicular to the longitude).

The small dial at 10 o'clock shows the distance from the center of the Earth to the center of the Moon (in thousands of kilometers).

The time is shown at the bottom left, including the day of month. The moonphase is shown at bottom right using the unique mechanical display of Emerald Chandra. Azimuth and altitude of the Moon are shown around that circular display with small red and blue hands.

Moonrise and moonset are shown in the top center.

The two thin red triangle hands on the main dial represent the "nodal" points of the Moon's orbit. These are the two points where the Moon's ecliptic latitude is zero, as it cycles above and below the plane defined by the Earth's orbit around the Sun. The position of each hand indicates the delta ecliptic longitude of that nodal point, so that when the black hand passes over a red nodal point marker, the ecliptic latitude is zero at that time. The nodal markers (unlike the nodal hands on Selene) make a full revolution approximately once per year, as the ecliptic longitude of the Sun makes a similar revolution.

No network access is required; only the device time and location are used.

More information is available at the listed website.