Tag Archives: large moon

What is so Super about a Super Moon?

August 10, 2014 just passed. It was the most recent Super moon. The term “Super moon” was coined by astrologers not astronomers and refers to a moon which is both full and also within 4 hours of its closest approach to earth.

The media gleefully report the super moon and show pictures of huge moons (many of which have been photo manipulated).  Here is the straight scoop on the subject. If you’re wondering whether that photo you’ve seen of the “too big to be true moon” has been doctored, we have an article on that.

Extreme SuperMoon [5_059193]

The most “Extreme” Supermoon of the Century occurred in 2012.  Here it was photographed in Yosemite approximately 15 minutes after it reached perigee.

What Makes the Moon Larger or Smaller When Seen from Earth?

Because the orbit of the moon around the earth is not circular, the distance from earth to the moon varies and thus the apparent (angular size) of the moon changes. Every lunar cycle the earth-lunar distance varies between its closest approach called perigee and its farthest distance, called apogee.  How big is the difference? The closest approach is 363,104 km (225,622 miles) and the farthest, 406,696 km (252,088 miles). 

What is the difference in apparent size?  At apogee, the moon is 22,293 km farther away or -5.8% smaller than an average moon.  At perigee the moon is 5.54% larger than the average moon. Comparing apogee and perigee moons, the difference is a maximum angular size difference of about 12%  The average angular size of the moon, by the way, is half of a degree or 30 minutes of arc. That angle is slightly smaller than the size of the nail on your little finger when held at arm’s length. Those of you with significantly mis-sized pinky nails or unusual arm length might want to find another object to measure with at arm’s length.

In short: You’d have to be a very keen observer to notice a 12% difference in size between a super moon and a “wimpy” (apogee) moon.

Because the moon is slowly spiraling away from earth eventually the perigee moon will grow smaller and smaller in apparent size until one day, we will no longer experience total solar eclipses. The perigee moon will be too small to cover the angular disk of the sun which also happens to be almost exactly one half of a degree. From that point on, all solar eclipses will be “annular” like this one in May, 2012. Had the moon been closer to the earth, this may have been a total solar eclipse.

Annular Eclipse Sequence [C_040079+5s]

 

 

How is a Full Moon Determined?

A full moon is defined as the moment in time when the sun, earth and moon are in syzygy. Syzygy is not only an interesting Scrabble(tm) word, but it defines when three bodies are in alignment. When the sun and moon are 180 degrees opposite one another relative to the Earth, we have syzygy which is the instance in which the moon is Full Moon. Many of us think of a “full moon” as that period during the month when the moon appears to be fully lit. That period lasts almost 70 hours, so we understand how reckoning a full moon as a moment in time is a bit confusing.

If you didn’t observe the August moon within 4 hours either side of when it was full, you did not see the super moon.  On the United States West Coast the super moon was not visible. Why? The moon set at 6:10 AM almost 5 hours before the moon was full. Those in Hawaii could just catch the super moon setting.  Those on the East Coast of the US had no chance at all. The whole super moon window occurred during the time the moon was not visible on the East Coast.

The Last (and Next) Visible Super Moons

If you missed the May, 2012 Extreme Super Moon (my term, photo above), you’ve missed the largest possible full moon for more than a century into the future. On May 5, 2012 fullness and perigee occurred within less than two minutes of one another.  But don’t fret.  The difference in size between the extreme super moon an the average super moon is too small to notice unless you measure carefully.  If you paid close attention you probably also noticed that the May 20, 2012 annular solar eclipse followed nearly half a lunar cycle after the May 5, 2012 super moon. That is not a coincidence! The moon was closest to us on May 5th so half of a lunar cycle away it must be farthest from us!

On August 10, 2014, full moon and perigee occurred within about 1 hour of each other. The next super moon is in September 8, 2014. The moon will not be as close to perigee at the moment when it becomes full, but the moment of full moon occurs at 9:38 PM PDT, just as the moon rises. It will be a true super moon!

Catch One Yourself

We plan to schedule a “Catching the Moon” Webinar well in advance. Stay tuned.  One complication is that the wonderful Photographer’s Ephemeris Tool will cease to work in desktop mode soon. It is being replaced with a browser version. While the tools is excellent, and we highly recommend it (and that you donate if you use it!) TPE still leaves some important bits of the puzzle unresolved – we will fill those in for you and give you a crack at our tool(s).

The moon caught between El Capitan and Half Dome - Actual size, no manipulation

The moon caught between El Capitan and Half Dome, Yosemite National Park – Actual size, no manipulation

Alignment (Part 1 of 2)

Sometimes alignment is everything. As an amateur astronomer at heart I am fascinated by the course of celestial bodies through the heavens and how they coincide with terrestrial features (is that strange?). Buildings, lighthouses, arches, and yes, observatories all beg to be photographed as they are kissed by the moon, the sun, or the Milky Way.

One example is shown in Photo 1, below. The moon is rising behind the Lick Observatory. The observatory is perched on the top of Mount Hamilton overlooking San Jose, California.  The photo was taken from the shore of Halls Valley Lake in Joseph D. Grant Park County Park. But it was not serendipitous – it was purposeful.  Days and weeks worth of planning were required.

Moon Lick [5_009717]

Photo 1: The full moon rises over Lick Observatory, Mount Hamilton, San Jose, California

An alignment of man-made artifacts and the moon occurred on the night of a total lunar eclipse.  It also was not completely accidental – but I can not claim I knew I would capture it:

Airplane Transits the Partially Eclipsed Moon

Photo 2: An airplane transits the partially eclipsed moon.

But sometimes the moon just happens to be in the right spot, as in this photograph by May Wong which captured the moon in an interesting alignment while hiking up a trail in Mission Peak Preserve.

Photo 3: (May Wong) The moon teed up on Mt. Allison's Tower

Many fascinating views of the sun and moon can be found in books by Harold Davis one particularly interesting example is “100 Views of the Golden Gate Bridge“.

Planning Moonshots

Ignoring happy accidents for a moment, getting the moon to align with some terrestrial object involves quite a bit of calculation. While there are some great tools to aid the lunar photographer (The Photographer’s Ephemeris, for example), it helps to understand why the moon is a difficult object to catch.  Starting with the first problem:

The Moon is BRIGHT

Jewel [C_029690]

Photo 4: Long exposure for details during a total eclipse - notice the few stars.

Indeed the moon is a very bright object as most people discover when they try to capture any of the details of the moon. Typically the full moon requires settings of f/9, ISO 100, and 1/100 of a second to preserve detail; but at night, those  settings result in everything else being a deep black, therefore to get moon details and foreground details there must be some illumination.  The best time is before sunrise or after sunset and more specifically the very best time is on the cusp between nautical twilight and civil twilight.  I will explain what those are in Part 2.  Of course the moon also makes planning harder by the changing daily illumination. In 29.53 days the moon completes one full cycle from new where the moon is in line with the sun and not illuminated; to full – opposite the sun in the sky and fully illuminated; and back to new. Surprisingly, however, the exposure needed to capture moon detail does not change very much until the moon becomes a slender sliver. When in the sliver phase longer exposures can capture moon detail in the darker (unlit) portions of the moon though this effort comes at the cost of blowing out detail from the lit edge.  In the extreme case, as when eclipsed (Photo 4) longer exposures are needed.

This brings us to the second problem:

The Moon’s Path through the Sky Changes Daily

As if the changing illumination were not enough the moon’s path through the sky  dramatically changes from day to day. At my latitude (39 degrees north) the moon rises about 42 minutes later each day.  The compass direction (azimuth) at which the moon rises and sets also changes significantly from day to day.   Capturing the moon near the horizon during twilight ALWAYS means attempting a shot of either a slender crescent moon or a full moon.  In most months at most 2 days near the full moon provide full moon capture opportunities. What about the other phases? During the first quarter, the moon is highest in the sky near sunset. During its last quarter the moon is highest in the sky at sunrise. So in short, at first and last quarter you have to shoot nearly straight up to get the moon.

NOTE: First quarter refers not to the amount of the moon that is lit – it is half lit – but to the phase. Similarly at last quarter the moon is also half lit.

Determining the rise and set times of the moon is not hard. Many sites feature the sun and moon rise times.  www.sunrisesunset.com is one site I like. sunrisesunset.com can generate a calendar for a whole month. With a little experience it is often enough to know what phase the moon is in. For me a calendar that does not feature moon phases is useless!

Once I choose which direction I will be shooting, I then know whether I must shoot near sunrise or sunset. Pigeon Point Lighthouse – my nemesis – is on the west coast. To capture the moon behind it the full moon must be setting – which means the sun is rising.  (It also means a 3:00 AM wake up to allow me time to drive to the coast!) Conversely when  attempting to capture the moon over the San Francisco Bay Bridge, the best viewing locations face east – meaning an evening (sunset) shot is best. One advantage to attempting the full moon is that the sun’s glow illuminates the face of the foreground whereas when shooting a crescent the sun and moon are on the same side of the sky so the foreground is in silhouette.

Now we face problem three:

The Moon is Tiny

In this wide angle shot, it is difficult to even see the moon! It’s there in the upper left, but with the 10mm lens the entire moon occupies about 467 pixels out of the 15,154,290 (15M) total pixels. That’s a paltry 0.03 percent of all the pixels in the image. Of course the moon is not tiny, it is very large but it is so far away that its angular size is 1/2 of a degree or about the width of your pinky finger at arms length.

When the Lights Go Down in the City [5_018683]

Photo 5: 20mm Focal length = tiny Moon... did you spot it?

Often my goal is to include a moon in a way that shows it large and well featured relative to the foreground. There is no practical way to get closer to the moon, so the way to make the moon larger in the frame is to use a telephoto lens (as in photo 1 and 2).

Putting the moon near some foreground element allows me to exploit the large moon phenomenon as shown in Photo 1. But it is not enough to use a telephoto lens – I must also be far enough away from the object in question so that the apparent (angular) size of the moon is nearly equal to the angular size of the foreground object. The proper distance can be measured with the pinky fingernail at arms length, or calculated with some trigonometry. In Part 2 I’ll supply a simple formula that works well. Meanwhile Figure 1 illustrates the challenges involved in positioning and “sizing” the moon relative to a foreground object.

Figure 1: Relative sizes of the moon based on distance from the foreground object. See notes.

NOTE: To keep the lighthouse the same size as shown in images A, B, and C above the focal length must be increased. Alternatively, using one fixed focal length pictures B and C can be cropped from a larger photo.

And there is another complication, too, depth of field. The longer the focal length the harder it is to keep the foreground and the background in focus. And one last complication:

Near The Horizon, Atmospheric Conditions have a Significant (Negative) Effect

Looking straight up there are about 50 kilometers of atmosphere to diminish the quality of a photo. Looking toward the horizon, that number is effectively 38 times as much! The sky must be clear of clouds and haze through the entire distance. And a more sinister thing occurs, too. The atmosphere bends the light. When objects like the sun or moon approach the horizon the atmospheric distortion can become quite noticeable as a vertically flattened object. And finally, due to refraction when the sun or moon appears to be setting, it in fact has already fallen below the horizon and remains visible only because of  refraction.  The take away here is that trying to capture a detailed moon at the horizon is not as effective as capturing the moon at least a few degrees above the horizon.

In Summary

To capture the moon near a terrestrial feature:

  • The moon’s current illumination must be managed.
  • The moon’s rising (or setting location) must be accurately calculated.
  • Exposures to capture moon detail require the right amount of foreground illumination (near twilight)
  • The location chosen must have an unobstructed view of the sky toward the desired direction.
  • To get a “big moon” it is necessary to get far enough away from the foreground to get the relative moon size as desired. If too close, depth of field problems arise.
  • A well supported telephoto lens is required.
  • Capturing a shot of the moon near the horizon means the atmosphere must be relatively clear of clouds, dust and haze.
  • Too low in the sky means there will be significant distortion from the atmosphere.

So there it is: all the complications that must be overcome in order to capture the moon. I just have not written HOW to overcome all those obstacles, that information is coming in the next installment.