Category Archives: Moon

Astronomy, Moon

When will the Proxigean Tides arrive between 2025-2030?

The Proxigean Tide, also called a King Tide, occurs when the Moon is at its closest point in its orbit to the Earth (perigee) and in its New or Full Moon phase as shown in this NOAA SciJinks figure. They also coincide with ‘supermoon’ full moons. At this time, which happens about once every 18 months, the moon’s tidal effect on Earth is maximum. Tidal gravitational forces vary as the third power of the distance between Earth and Moon, so even a small difference in distance can translate into a big effect. The orbit of the moon varies from a distance of 356,500 to 406,700 kilometers with an average distance near 380,000 kilometers. The variation between the maximum and minimum distances results in tidal force changes of a factor of 1.2 times the average tidal forces. The times when this will happen often coincide with major coastal flooding events.

The perigee/apogee and lunar phase calculator at Fourmilab provides a quick means of predicting when these tides will occur. The closest perigee and most distant apogee of the year are marked with “++
” if closer in time to full Moon or “–” if closer to new Moon. Other close-to-maximum apogees and perigees are flagged with a single character, again indicating the nearer phase. Following the flags is the interval between the moment of perigee or apogee and the closest new or full phase; extrema cluster on the shorter intervals, with a smaller bias toward months surrounding the Earth’s perihelion in early January. “F” indicates the perigee or apogee is closer to full Moon, and “N” that new Moon is closer. The sign indicates whether the perigee or apogee is before (“−”) or after (“+”) the indicated phase, followed by the interval in days and hours. Scan for plus signs to find opportunities where the Moon is full close to perigee. The most extreme events based on the time between perigee and Full/New being less than 5 hours:

Date            Perigee      Phase

3/10/2024        356,893      New
10/17/2024       357,172      Full
4/27/2025        357,118      New
11/5/2025        356,832      Full
6/14/2026        357,195      New
12/24/2026       356,649      Full
8/2/2027         357,361      New
2/10/2028        356,677      Full
9/18/2028        357,047      New
3/30/2029        356,664      Full
11/5/2029        356,899      New
3/17/2030        357,017      Full
12/24/2030       356,924      New

New Moon is pretty bad because both the Sun and the Moon are on the same side of the Earth, and with the Moon near its closest point to the Earth, the tide- making potential is highest. The date for the smallest perigee distance is December 24, 2026 when the Full ‘Super Moon’ will have its largest angular diameter of 2009 arc-seconds or 0.56 degrees.

Astronomy, Moon

How did the Indigenous Peoples of North America name the Full Moons?

The Harvest Moon goes by many other names. (Credit:Wikipedia). There are several lists of these to be found across the WWW. The Old Farmers Almanac has one such list based upon the Algonquin names. A full Moon name used by one tribe might differ from one used by another tribe for the same time period, or be the same name but represent a different time period. The name itself was often a description relating to a particular activity/event that usually occurred during that time in their location.

Month               ALGONQUIN               OJIBWA
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1. January      WOLF MOON          GREAT SPIRIT MOON
2. February     SNOW MOON          SUCKER SPAWNING MOON
3. March        SAP MOON    MOON OF THE CRUST ON THE SNOW
4. April        SEED MOON          SAP RUNNING MOON
5. May          FLOWER MOON          BUDDING MOON
6. June         STRAWBERRY MOON          STRAWBERRY MOON
7. July       BUCK MOON          MIDDLE OF THE SUMMER MOON
8. August       STURGEON MOON          RICE-MAKING MOON
9. September      CORN MOON          LEAVES TURNING MOON
10. October      RAVEN MOON          FALLING LEAVES MOON
11. November     HUNTER MOON          ICE FLOWING MOON
12. December     COLD MOON          LITTLE SPIRIT MOON

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Colonial Americans adopted some of the Native American full Moon names and applied them to their own calendar system (primarily Julian, and later, Gregorian). For example, the Harvest Moon is associated by the colonists with the full moon nearest the Autumnal Equinox on September 21.

It is also worth pointing out that New Moons also have their own names, though limited in number and refer to the second new moon in a given month. These are called the Secret Moon, Finder’s Moon, Spinner Moon and Black Moon.

Contrary to Creedence Clearwater Revival, there is no such thing as a ‘Bad Moon’.

Here are the dates and times for the next series of named moons for 2017:

Month               ALGONQUIN               
--------------------------------------------------------------------------
1. January               WOLF MOON       January 12,      6:34 am EDT   
2. February              SNOW MOON       February 10,     7:33 pm EDT  
3. March                  SAP MOON       March 12,       10:54 am EDT  
4. April                 SEED MOON       April 11,        2:08 am EDT 
5. May                 FLOWER MOON       May 10,          5:42 pm EST  
6. June            STRAWBERRY MOON       June 9,          9:10 am EST 
7. July                  BUCK MOON       July 9,         12:07 am EST 
8. August            STURGEON MOON       August 7,        2:11 pm EST   
9. September             CORN MOON       September 6,     3:03 am EST   
10. October             RAVEN MOON       October 5,       2:40 pm EDT 
11. November           HUNTER MOON       November 4,      1:23 am EDT  
12. December             COLD MOON       December 3,     10:47 am eDT        
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There is also the famous Blue Moon, which is the second full moon in a given month. The last Blue Moon occurred on May 21, 2016, and the next one will be on January 31, 2018. In four or five years per century, there are two Blue Moons. The first Blue Moon always occurs in January. The second occurs predominantly in March. In the 10,000 years starting with 1600, this is true in 343 out of 400 cases, or 86 per cent of the time. In 37 cases (or 9 per cent), the second Blue Moon is in April. In the remaining 20 cases (5 per cent) it is in May.

In January 1999 we had a Blue Moon in January and one in March. The next event will happen in 2018 also in January and March. Then you will have to wait until 2037 for the Blue Moons in January and March.

Because the events of Halloween Eve are heightened by having a Full Moon in the sky, this Raven Moon on October 31 will next occur in the year 2020.

Astronomy, Moon

Why does the moon rise 50 minutes later each day?

Here is a simulation of the moon on nine consecutive nights at the same local time. This image shows the positions of the sun and moon with respect to the stars over a nine-day period. The yellow line is the ecliptic, from which the moon never strays by more than about five degrees. (The sizes of both the sun and moon are exaggerated for emphasis.) Courtesy Daniel Schroder.

Imagine the following line as a part of the Moon’s orbital path across the sky from west to east. The moon travels from West to East across the sky, and makes one full journey with respect to the stars every 27.3 days (a Sidereal Month):

East……………….M……………….West

Now, if it takes 27.3 days to travel once around Earth, the moon must travel 360 degrees/27.3 days = 13.18 degrees/day to the East. This means on the next night, the moon is located 13.18 degrees to the East from last night’s location:

East………….M…………………….West

This means that Earth has to turn an extra 13.18 degrees so that tonight’s moon is in the same sky position as last night’s moon. If last night the moon was just at the eastern horizon, tonight at the same time it is 13.18 degrees below the eastern horizon.

Now, how long does it take Earth to turn 13.18 degrees? Well, in 24 hours, it turns 360 degrees, so in (13.18/360)x 24 = 0.88 hours or 52.7 minutes, the sky rotates the extra 13.18 degrees.

Why do we use the time of the Sidereal month instead of the Synodic month which is 29.53 days? Because we are interested in the moon’s relation to its position relative to the background sky (Sidereal) not whether it is in the same orientation with respect to the Sun and Earth. A Synodic month separates one New Moon from the next New Moon, or any corresponding similar lunar phases on any two cycles. If we were to use the Synodic month, we would get a lunar shift of 12.1 degrees per day, and that the moon would rise 48.7 minutes later each night. The average of the two is 50.7 minutes. The difference between the two is 4 minutes, which is just the amount that the Sun has moved to the East in ITS motion along the sky. This emphasizes that Sidereal time does not depend on the location of the Sun, but Synodic does!

Astronomy, Gravity, Moon, Planets

Why are there no ocean tides at the equator?

A typical scene on North Seymour in the Galapagos Islands. (Credit:Wikipedia-David Adam Kess). In general, tides along continental shores near the equator are much less violent than elsewhere.

Tides are a very complex phenomenon. For any particular location, their height and fluctuation in time depends to varying degrees on the location of the Sun and the Moon, and to the details of the shape of the beach, coastline, coastline depth and prevailing ocean currents. Here is a figure that shows the difference between high tide and low tide around the world.

Newton’s explanation is that, when you calculate the difference in gravity between Earth and moon at each point on the surface of Earth, you get the customary graph shown here:

This is also the shape of the ‘equipotential surface’ where mass would be in equilibrium and instantaneously ‘at rest’. There are two gravitational tides: The Body Tide and the Water Tide. The Body Tide is the response of the solid Earth to this gravitational distortion in the solid rock of Earth. The lunar body tide has a height of 0.3 meters relative to the unstressed shape of Earth while the solar body tide is about half this high. The water tides are far higher because water is lower density than rock and is free to flow around Earth’s surface with lower inertia than rock. Water tide heights can exceed 10 meters!

You would think that the solid body tide would flex the ground so severely that pipes, railroad tracks and other systems would flex and break over time. The good news is that the scale of this distortion is continent-spanning as the figure below shows.

So what does this all have to do with whether tides are found at the Equator?

Although Newton gave us the basic gravitational theory for solid body tides, his application of this theory to the behavior of water was not correct in detail. The French mathematician Laplace used Newton’s gravitational theory, but realized that its application to water tides had much more to do with the gravitational forcing of various water oscillations. Water oscillations, treated as a harmonic system with many different resonant frequencies is a much more powerful description of the details of water tides on Earth. When you combine the main lunar water tide and the solar tides acting on a complex shaped layer of water along Earth’s surface, what you get is a very different pattern of high and low water tides shown in this figure.

This figure created by Dr. Richard Ray/Space Geodesy branch, NASA/GSFC, shows the M2 lunar tidal constituent. Amplitude is indicated by color, and the white lines are cotidal differing by 1 hr. The curved arcs around the amphidromic points show the direction of the tides, each indicating a synchronized 6 hour period. Note that this response of ocean water has virtually nothing to do with the simple two-bulges, gravitational stress pattern expected from Newton’s calculation above.

So are there water tides at the Equator? Yes there are, and in fact the only locations that have very weak tides are near the poles!

Astronomy, Gravity, Moon

Could you explain what causes the Moon’s synchronous rotation?

At the top of this article is a figure that shows how deformed the moons shape is from a perfect sphere based on orbital data from the Lunar Orbiter spacecraft. The topography of the Moon referenced to a sphere with a radius of 1737.4 kilometers. Data were obtained from the Lunar Orbiter Laser Altimeter (LOLA) that was flown on the mission Lunar Reconnaissance Orbiter (LRO). The color coded topography is displayed in two Lambert equal area images projected on the near and far side hemispheres.

The tidal force of the Earth’s gravitational field raises solid-body tides on the Moon causing the Moon to be deformed into a non-spherical body resembling a football. The magnitude of this effect is about 20 times the solid-body tide caused by the Moon upon the Earth which is about 20×20 centimeters or 4 meters. When the Moon was first formed, it was closer to the Earth than it is now, so the tidal amplitude was quite a bit greater, moreover, the Moon was molten and so it responded even more strongly to the tidal deformation imposed upon it by the Earth’s gravitational field. As a result, the shape of the Moon is very far from being spherical. The Moon was originally rotating faster than it is now so that 3-4 billion years ago it was not orbiting the Earth as fast as it was rotating about its axis.

Over the years, however, the gravitational tidal forces acting upon the non-spherical body of the Moon have modified its non-spherical shape, and caused a systematic dissipation of the Moon’s rotational energy via friction. It costs a lot of energy to deform the Moon, and this energy is lost through the internal friction of rock rubbing against rock within the Moon to raise the solid body tides. Because the Moon may already have solidified into a football-shaped non-spherical body, there is a portion of the Moon that is always slightly closer to the Earth than other portions of the Moon. This becomes a ‘handle’ that the gravitational field of the Earth can ‘grab onto’ to apply a slightly greater force upon the Moon that at other times during the lunar orbit around the Earth. A similar deformity exists in Mercury which has aided the Sun in synchronizing Mercury into a 2:3 spin-orbit resonance. For the Moon, and the larger satellites of the other planets, a similar deformity leads to a 1:1 resonance so that the same side of the satellite always faces the planet.

So, a combination of the Moon’s initial deformation when it was molten and solidified in the Earth’s tidal gravitational field, together with the on-going tidal deformation, leads to a preferred orientation to the Moon in its orbit which the system relaxes to over billions of years.