We tend to take the Moon for granted but it shares a unique history with Earth. Shortly after its formation 4.5 billion years ago, “proto-Earth” collided with a Mars-sized object called Theia. Much of “proto-Earth” and Theia merged to become our Earth, but the impact also ejected a large amount of material into space. Some of it coalesced to become the Moon (see: Giant Impact Hypothesis).
The Moon’s orbit stabilizes the axial tilt of Earth, preventing it from undergoing chaotic variations that would lead to catastrophic changes in climate. And the daily rise and fall of the Moon-induced tides has left an indelible imprint on Earth. Some scientists even argue whether life on Earth would be possible without the influence of the Moon (see: Without the Moon, Would There Be Life on Earth?).
With this big picture in mind, we gain a new appreciation for the Moon as we watch its phases, cycles, and motions during 2017.
As the Moon orbits Earth, its changing geometry with respect to the Sun produces the Moon’s characteristic phases (New Moon, First Quarter, Full Moon and Last Quarter). One orbit of the Moon relative to the Sun (the synodic month) has a mean duration of 29.5306 days (29 days 12 hours 44 minutes).
Moon Phases for 2017 (GMT) New Moon First Quarter Full Moon Last Quarter ------------- ------------- ------------- ------------- Jan 5 19:47 Jan 12 11:34 Jan 19 22:14 Jan 28 00:07 Feb 4 04:19 Feb 11 00:33n Feb 18 19:33 Feb 26 14:58A Mar 5 11:32 Mar 12 14:54 Mar 20 15:58 Mar 28 02:57 Apr 3 18:39 Apr 11 06:08 Apr 19 09:57 Apr 26 12:16 May 3 02:47 May 10 21:43 May 19 00:33 May 25 19:44 Jun 1 12:42 Jun 9 13:10 Jun 17 11:33 Jun 24 02:31 Jul 1 00:51 Jul 9 04:07 Jul 16 19:26 Jul 23 09:46 Jul 30 15:23 Aug 7 18:11p Aug 15 01:15 Aug 21 18:30T Aug 29 08:13 Sep 6 07:03 Sep 13 06:25 Sep 20 05:30 Sep 28 02:54 Oct 5 18:40 Oct 12 12:25 Oct 19 19:12 Oct 27 22:22 Nov 4 05:23 Nov 10 20:37 Nov 18 11:42 Nov 26 17:03 Dec 3 15:47 Dec 10 07:51 Dec 18 06:31 Dec 26 09:20
The table above lists the date and time of the Moon’s phases throughout 2017. The time of each phase is given in Greenwich Mean Time or GMT (a.k.a. Universal Time or UT). A table of the Moon’s phases covering 100 years on AstroPixels.com can be found at Moon’s Phases – 21st Century (GMT). Similar 100-year tables for other time zones include Eastern Standard Time (EST), Central Standard Time (CST), Mountain Standard Time (MST), and Pacific Standard Time (PST). To convert GMT to other time zones, visit Time Zones.
What surprises many people is that the length of the synodic month (period from New Moon to New Moon) can vary by more than 6 hours from its mean value of 29.5306 days (29 days 12 hours 44 minutes). The table below gives the date of New Moon, the length of the synodic month, and the difference from the synodic month’s mean value for every synodic month in 2017. For instance, the fifth synodic month of 2017 (beginning May 25) is 5 hours 58 minutes shorter than the mean while the twelfth synodic month (beginning Dec 18) is 7 hours 3 minutes longer than the mean.
Synodic Months for 2017 Date/Time of Length of Difference from New Moon (GMT) Synodic Month Mean Month ------------------ ------------- ----------- 2017 Jan 28 00:07 29d 14h 51m +02h 07m 2017 Feb 26 14:58 29d 11h 59m -00h 45m 2017 Mar 28 02:57 29d 09h 19m -03h 25m 2017 Apr 26 12:16 29d 07h 28m -05h 16m 2017 May 25 19:44 29d 06h 46m -05h 58m shortest 2017 Jun 24 02:31 29d 07h 15m -05h 29m 2017 Jul 23 09:46 29d 08h 45m -03h 59m 2017 Aug 21 18:30 29d 11h 00m -01h 44m 2017 Sep 20 05:30 29d 13h 42m +00h 58m 2017 Oct 19 19:12 29d 16h 30m +03h 46m 2017 Nov 18 11:42 29d 18h 48m +06h 04m 2017 Dec 18 06:30 29d 19h 47m +07h 03m longest
What causes these variations? The explanation involves the Moon’s elliptical orbit and its orientation with respect to the Sun during any given month. If New Moon occurs when the Moon is nearest to Earth (perigee), then the synodic month is shorter than normal. On the other hand, if New Moon occurs when the Moon is farthest from Earth (apogee), then the synodic month is longer than normal. Furthermore, the orientation of the Moon’s ellipse-shaped orbit slowly rotates in space with a period of about 18 years. A more detailed discussion on this topic can be found at Moon’s Orbit and the Synodic Month (EclipseWise.com). You can also find the duration of every synodic month this century at Length of the Synodic Month: 2001 to 2100 (AstroPixels.com).
The time it takes for the Moon to orbit once with respect to its perigee is known as the anomalistic month. Its average length is 27.55455 days (27 days 13 hours 19 minutes), which is nearly 2 days less than the synodic month. The actual length can vary by several days due to the gravitaional effects of the Sun on the Moon’s elliptical orbit. The table below gives the date and time of every perigee and apogee of the Moon during 2017. The Moon’s distance (in kilometers) is also given. The ‘m’ or ‘M’ appearing next to a date indicates the minimum or maximum distance, respectively, for the year. A table listing details of every perigee and apogee this century can be found at Perigee and Apogee: 2001 to 2100 (AstroPixels.com)
Perigee & Apogee for 2017 Date/Time of Distance Date/Time of Distance Perigee (GMT) (km) Apogee (GMT) (km) ------------- ------ ------------- ------ Jan 10 06:07 363242 Jan 22 00:14 404913 Feb 06 13:59 368817 Feb 18 21:14 404376 Mar 03 07:24 369065 Mar 18 17:25 404651 Mar 30 12:39 363855 Apr 15 10:05 405478 Apr 27 16:18 359325 May 12 19:51 406212 May 26 01:23 357210 m Jun 08 22:21 406402 Jun 23 10:49 357938 Jul 06 04:27 405934 Jul 21 17:09 361238 Aug 02 17:55 405026 Aug 18 13:14 366129 Aug 30 11:25 404307 m Sep 13 16:04 369856 M Sep 27 06:49 404342 Oct 09 05:51 366858 Oct 25 02:25 405151 Nov 06 00:09 361438 Nov 21 18:52 406132 Dec 04 08:42 357496 Dec 19 01:27 406605 M
Because the Moon orbits Earth in about 29.5 days with respect to the Sun, its daily motion against the background stars and constellations is quite rapid, averaging 12.2° per day. A table giving the Moon’s daily celestial coordinates throughout the year can be found at Moon Ephemeris for 2017 (AstroPixels.com). This table lists many other details about the Moon including its daily distance, apparent size, libration, phase age (days since New Moon) and the phase illumination fraction.
When a Full Moon occurs within 90% of the Moon’s closest approach to Earth in a given orbit, it is called a Perigean Full Moon or more commonly a Super Moon. The Full Moon then appears especially big and bright because it subtends its largest apparent diameter as seen from Earth. The table below lists the Perigean Full Moons (Super Moons) occurring in 2017.
Perigean Full Moons (Super Moons) for 2017 Full Moon Distance Diameter Relative (GMT) (km) (arc-min) Distance Jan 12 11:34 366880 32.57 0.913 Nov 04 05:23 364004 32.83 0.941 Dec 03 15:47 357987 33.38 0.990 closest
The Relative Distance listed in the Super Moon table expresses the Moon’s distance as a fraction between apogee (0.0) and perigee (1.0). For more information on Super Moons and a complete list of them for this century, see Full Moon at Perigee (Super Moon): 2001 to 2100 (AstroPixels.com).
Besides its obvious phases, the Moon also undergoes some additional extremes in its orbit including: Perigee and Apogee, Ascending/Descending Nodes, and Lunar Standstills. Each of these AstroPixels links covers lunar phenomena for the entire 21st Century.
One of the first projects I tackled upon completing Bifrost Observatory in 2010 was to photograph the Moon’s phases every day for a complete month. Of course, the weather doesn’t always cooperate (even from sunny Arizona) so it actually took several months to complete the project. You can see the results at the Moon Phases Gallery. Clicking on any of the thumbnail pictures will give you an enlarged image with complete technical details. You can also visit Moon Phases Mosaics to see composites showing the Moon’s phases over a complete synodic month.
The NASA/Goddard Scientific Visualization Studio has used image data from the Lunar Reconnaissance Orbiter (LRO) mission to create clever animations of the Moon’s ever changing phases and librations in 2017. The example below illustrates the Moon’s phase and libration at hourly intervals throughout 2017, as viewed from the northern hemisphere. Each frame represents one hour.
And not to be accused of northern hemisphere chauvinism, here is a version as seen from the southern hemisphere.
Besides presenting the Moon’s phase and apparent size, these videos show the Moon’s orbital position, sub-Earth and subsolar points, distance from the Earth at true scale, and labels of craters near the terminator. As the Moon orbits Earth, it appears to wobble and tip on its axis. This motion is called libration and it allows us to see about 59% of the Moon’s surface (see Libration (EarthSky)). The major cause of libration is due to our changing line of sight because of the Moon’s elliptical orbit.
Ernie Wright of the NASA Scientific Visualization Studio has also used LRO data to create a web tool called Dial-A-Moon. Enter the month, day and hour and Dial-A-Moon will generate a visualization of the Moon showing the correct phase and libration for that instant during 2017 (see Moon Phase and Libration, 2017).
Finally, what discussion of the Moon would be complete without mentioning eclipses in 2017? There are two eclipses of the Moon. The first is a deep penumbral eclipse on February 11, which is visible from both the Eastern and Western Hemispheres. Penumbral eclipses are rather subtle events and often transpire without any notice (see: Visual Appearance of Penumbral Lunar Eclipses). The second lunar eclipse is partial on August 7 and is visible from the Eastern Hemisphere.
There are also two solar eclipses in 2017. The first is an annular eclipse on February 26. The annular phase of the eclipse is visible from Chile, Argentina, the south Atlantic, Angola, Democratic Republic of the Congo, and Zambia.
The second solar eclipse of the year is the long awaited Great American Total Solar Eclipse on August 21. This is the first total solar eclipse visible from the continental USA in 38 years. For complete details on this highly anticipated event, see: 2017 Total Solar Eclipse (EclipseWise.com).
For more details on all these events, see Eclipses During 2017 (EclipseWise.com).
For those who are new to Moon watching, many are surprised that the entire Moon can often be seen during the crescent phase. The pale glow illuminating the unlit part of a crescent Moon is light reflected from Earth and it’s called earthshine. The time-lapse movie above captures earthshine during moonset back one evening in May 2015. Read more about earthshine in this Earth&Sky article.
Watching the Moon’s phases wax and wane as well as the occasional lunar eclipse can best be enjoyed with the naked eye and binoculars. And you don’t even need a dark sky since the Moon is easily visible from the heart of brightly lit cities.
The Moon phases and lunar phenomena discussed here were all generated with computer programs I’ve written (THINK Pascal and FORTRAN 90 running on a Macintosh G4 and MacBook Pro) using Astronomical Algorithms (Jean Meeus).
- Reference Links
- Phases of the Moon: 2001 to 2100 (Greenwich Mean Time)
- Phases of the Moon: 2001 to 2100 (Eastern Standard Time)
- Phases of the Moon: 2001 to 2100 (Central Standard Time)
- Phases of the Moon: 2001 to 2100 (Mountain Standard Time)
- Phases of the Moon: 2001 to 2100 (Pacific Standard Time)
- Six Millennium Catalog of Phases of the Moon: -1999 to 4000 (2000 BCE to 4000 CE)
- Moon Ephemeris for 2017
- Length of the Synodic Month: 2001 to 2100
- Full Moon at Perigee (Super Moon): 2001 to 2100
- Perigee and Apogee: 2001 to 2100
- Ascending/Descending Nodes: 2001 to 2100
- Lunar Standstills: 2001 to 2100
- Moon Phases Photo Gallery
- Moon Phases Mosaics
- Eclipses During 2017 (EclipseWise.com) – detailed information, maps and tables
- 2017 Total Solar Eclipse (EclipseWise.com)
- Solar and Lunar Eclipses (EclipseWise.com) – a web site containing predictions for thousands of solar and lunar eclipses
- Eclipses and the Moon’s Orbit (EclipseWise.com) – how the Moon’s orbital cycles are related to eclipses