Dominic Ford, Editor
From the Moon feed
The Moon will be prominent in the evening sky, setting around midnight.
From Seattle , it will become visible at around 21:32 (PST) as the dusk sky fades, 40° above your south-western horizon. It will then sink towards the horizon, setting 4 hours and 48 minutes after the Sun at 01:54.
At this time in its monthly cycle of phases, it appears almost exactly half illuminated.
Observing the Moon at first quarter
As it progresses through this cycle, it is visible at different times of day. At last quarter, it rises at around midnight, appears high in the sky by dawn, and sets at around midday. Click here for more information about the Moon's phases.
The period when the Moon shows half phase is ideal for observing the Moon with a pair of binoculars or a small telescope. The border between the light and dark portions of the Moon's disk is the best place to look for detail on its surface, because along this line, the Moon's surface is illuminated at a very shallow angle. As a result, mountains and crater rims cast long shadows which are very easy to see. An observer on the Moon would see the Sun on the horizon, casting long shadows just like the ones we see on Earth at sunrise and sunset.
At first quarter and last quarter, when the terminator line is down the middle of the Moon, it is best presented for view, without any foreshortening.
Although the Moon passes first quarter every month, it is more favourably placed in the early evening sky at some times of year than others.
It appears high up in the evening sky around the spring equinox, but much lower towards the horizon around the autumn equinox.
This is because it always lies close to a line across the sky called the ecliptic. This marks the flat plane in space in which all of the planets circle the Sun. It is the line through the zodiacal constellations that the Sun follows through the year.
The altitude at which the Moon appears above the horizon at sunset depends how steeply the line of the ecliptic is inclined to the horizon. If the plane of the ecliptic meet the horizon at a very shallow angle, the Moon will rise or set along a line which is almost parallel to the horizon, and a large separation from the Sun along this line would still only correspond to a very low altitude in the sky.
The inclination of the ecliptic plane to the horizon at Seattle varies between 65° (sunset at the spring equinox) and 18° (sunset at the autumn equinox). On June 17, the ecliptic is inclined at 38° to the western sunset horizon, as shown by the yellow line in the planetarium view above, meaning that on this occasion the Moon is very favourably placed for viewing from Seattle.
The Moon's position
At the moment it reaches first quarter, the Moon's distance from the Earth will be 374,000 km. Its exact position will be as follows:
|Object||Right Ascension||Declination||Constellation||Angular Size|
The coordinates above are given in J2000.0.
|The sky on 16 December 2018|
9 days old
All times shown in PST.
The circumstances of this event were computed using the DE405 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).
This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.
|17 Jun 1983, 12:47 PDT||– Moon at First Quarter|
|25 Jun 1983, 01:33 PDT||– Full Moon|
|03 Jul 1983, 05:13 PDT||– Moon at Last Quarter|
|10 Jul 1983, 05:19 PDT||– New Moon|