British Astronomical Association
Supporting amateur astronomers since 1890

Secondary menu

Main menu

BAA Observing calendar

New Moon

Monday, 2017, December 18 - 06:32

Dominic Ford – originally published on In-The-Sky.org

The Moon will pass close to the Sun and become lost in the Sun's glare for a few days.

The Moon's orbital motion carries it around the Earth once every four weeks, and as a result its phases cycle from new moon, through first quarter, full moon and last quarter, back to new moon once every 29.5 days.

This motion also means that the Moon travels more than 12° across the sky from one night to the next, causing it to rise and set nearly an hour later each day. Click here for more information about the Moon's phases.

At new moon, the Earth, Moon and Sun all lie in a roughly straight line, with the Moon in the middle, appearing in front of the Sun's glare. In this configuration, we see almost exactly the opposite half of the Moon to that which is illuminated by the Sun, making it doubly unobservable because the side we see is almost entirely unilluminated.

Over coming days, the Moon will become visible in the late afternoon and dusk sky as a waxing crescent, setting an hour later each evening. By first quarter, in a week's time, it will be visible until around midnight. The times below are given in London local time:

Date Sun
sets at
Moon
sets at
Altitude of Moon
at sunset
Direction of Moon
at sunset
18 Dec 2017 15:47 16:24 south-west
19 Dec 2017 15:47 17:09 south-west
20 Dec 2017 15:48 18:00 11° south-west
21 Dec 2017 15:48 18:56 16° south-west
22 Dec 2017 15:49 19:57 20° south
23 Dec 2017 15:49 21:00 24° south
24 Dec 2017 15:50 22:04 27° south

At the moment of closest approach, it will pass within 3°53' of the Sun, in the constellation Sagittarius. The exact positions of the Sun and Moon will be:

Object Right Ascension Declination Constellation Angular Size
The Moon 17h44m10s -19°29' Sagittarius 29'23"
Sun (centre) 17h43m -23°23' Sagittarius 32'30"

The coordinates above are given in J2000.0.


The details of this observing event were provided courtesy of In-The-Sky.org

.