Moon in 2017

Moon Phases Mosaic

A mosaic made from 9 individual photos of the Moon captures its phases over one synodic month. For complete details about this image, see Moon Phases Mosaic. The individual images included in this composite can be found in the Moon Phases Gallery. For more composites, see Moon Phases Mosaics. Photo copyright 2012 by Fred Espenak.

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.

Moonrise on 2015 January 05 from Portal, AZ. Copyright 2015 by Fred Espenak.

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.

The most recent Perigean Full Moon (Full Moon near Perigee or closest point to Earth) took place on Dec. 13, 2016. The media loves to call this a “Super Moon” but I prefer to call it the less sensational “Perigean Full Moon” or “Full Moon near perigee”. Whatever you call it, it’s a chance to take a moment and marvel at our beautiful natural satellite. The photo below was taken from my driveway in Portal, AZ as the Moon rose above the Peloncillo Mountains of New Mexico. I was hoping for a completely clear sky but the clouds actually added an appealing element to the scene. Copyright 2016 by Fred Espenak.

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.

Moon Phases Mosaic

As the Moon orbits Earth, its changing geometry with respect to the Sun produces the characteristic phases. This composite image is a mosaic made from 25 individual photos of the Moon and illustrates its phases over one synodic month. For complete details about this image, see Moon Phases Mosaic. The individual images included in this composite can be found in the Moon Phases Gallery. For more composites, see Moon Phases Mosaics. Photo copyright 2012 by Fred Espenak.

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 total solar eclipse of August 21, 2017 is the first total eclipse visible from the continental USA since 1979. For more information see the special EclipseWise web page on the 2017 eclipse.

The total solar eclipse of August 21, 2017 is the first total eclipse visible from the continental USA since 1979. For more information see the special EclipseWise web page on the 2017 eclipse.

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).

Moonset: Crescent Moon & Earthshine from Portal, AZ. Copyright 2015 by Fred Espenak.

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).

Fred Espenak


Events for January 2017

The following table gives the date and time of important astronomical events for January 2017.

The time of each event is given in Greenwich Mean Time or GMT (a.k.a. Universal Time or UT). To convert GMT to Eastern Standard Time (EST) just subtract 5 hours. To convert GMT to other time zones, visit Time Zones. Some of the astronomical terms used in the calendar are explained in Definitions.

 Date    GMT   Astronomical Events for January 2017
------  -----  --------------------------------------------
        (h:m)
Jan 02  09:20  Venus 1.9°S of Moon
Jan 02  18:14  Moon at Descending Node 
Jan 03  06:47  Mars 0.2°S of Moon: Occultation
Jan 03  14     Quadrantid Meteor Shower
Jan 04  15     Earth at Perihelion: 0.98331 AU
Jan 05  19:47  FIRST QUARTER MOON 
Jan 09  09     Mercury 6.7° of Saturn
Jan 09  14:07  Aldebaran 0.4°S of Moon
Jan 10  06:07  Moon at Perigee: 363242 km
Jan 12  11:34  FULL MOON 
Jan 12  13     Venus at Greatest Elongation: 47.1°E
Jan 13  13:59  Beehive 3.9°N of Moon
Jan 15  04:07  Regulus 0.9°N of Moon
Jan 15  10:44  Moon at Ascending Node 
Jan 19  05:26  Jupiter 2.7°S of Moon
Jan 19  10     Mercury at Greatest Elongation: 24.1°W
Jan 19  22:14  LAST QUARTER MOON 
Jan 22  00:14  Moon at Apogee: 404913 km
Jan 24  10:37  Saturn 3.6°S of Moon
Jan 26  00:46  Mercury 3.7°S of Moon
Jan 28  00:07  NEW MOON 
Jan 29  22:21  Moon at Descending Node 
Jan 31  13:11  Jupiter 3.5°N of Spica
Jan 31  14:34  Venus 4.1°N of Moon

As the events above transpire, I will post photographs of some of them at Recent Images.

Astronomical events calendars for complete years and for eight time zones are available through the links below.

Time Zones Calendars of Astronomical Events
Greenwich Mean Time 2017 2018 2019 2020 2021 2022
Atlantic Standard Time 2017 2018 2019 2020 2021 2022
Eastern Standard Time 2017 2018 2019 2020 2021 2022
Central Standard Time 2017 2018 2019 2020 2021 2022
Mountain Standard Time 2017 2018 2019 2020 2021 2022
Pacific Standard Time 2017 2018 2019 2020 2021 2022
Alaska Standard Time 2017 2018 2019 2020 2021 2022
Hawaii Standard Time 2017 2018 2019 2020 2021 2022

For additional years, see Calendars of Astronomical Events.

The total solar eclipse of August 21, 2017 is the first total eclipse visible from the continental USA since 1979. For more information see the special EclipseWise web page on the 2017 eclipse.

The total solar eclipse of August 21, 2017 is the first total eclipse visible from the continental USA since 1979. For more information see the special EclipseWise web page on the 2017 eclipse.

The astronomical highlight of 2017 is the Great American Total Solar Eclipse on August 21. This is the first total 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 information on all solar and lunar eclipses this year, see: Eclipses During 2017.

The Calendars of Astronomical Events were all generated by a computer program I wrote (with THINK Pascal running on a Macintosh G4) using Astronomical Algorithms (Jean Meeus).

Fred Espenak