Many folks would like to see us back on the Moon and developing its resources.

Wednesday, March 22, 2006

Good day

Lunar Standstill - or - is seeing believing?

Paul Slootweg sent the note below about the Moon being low in the horizon and probably thought to be seen as large.

He has two reference links which I have expanded some below his post.
(some other link that may be of interest as well)

So is what you see really there?

I took a picture once of the Moon in a day lit sky. Looked so big and yet the camera image is just a small ball in the sky.

Our senses give us information about the outside world and our computation about the information received, is what we perceive our reality to be.

There is a slight delay in getting the information processed but never mind.
For the most part we hit the brakes soon enough, maybe.

Our learned filters screen the information and help us make judgments about whether to take flight or fight.

We compare what is perceived with what is expected to be seen.

Which is remembered, the seen or the expected?

A solar eclipse is coming up. The Moon will be between Earth and the Sun.
For now it is big enough to block the Sun. Some day, as it moves slowly away from Earth, it will not appear big enough to block the Sun totally.

Check out the Moon and reflect on whether we can believe everything we see (and hear). :-)

Thanks for looking up with me.

Larry Kellogg
Web Site:
RSS link:
Paul Slootweg sent this info. - LRK -
There's been no mention of it so far, but today sees the Moon the lowest on the horizon for 19 years.

This must be a good time to see the Moon Illusion.

Thanks for keeping us looking up :)
Lunar standstill
>From Wikipedia, the free encyclopedia

At the major lunar standstill, which takes place every 18.6 years, the range of the declination of the Moon reaches a maximum. As a result, at high latitudes, the Moon appears to move in just two weeks from high in the sky to low on the horizon. This time appears to have had special significance for the Bronze Age societies who built the megalithic monuments in Britain, and it also has significance for some neo-pagan religions. The next major lunar standstill occurs in 2006.

Informal explanation

As the Earth spins on its axis, the stars above us appear to move in circles. It appears to us as if all the stars are fixed in a great sphere surrounding us. In the same way that we measure positions on the earth using latitude and longitude, we measure positions of stars on this sphere in Right Ascension (equivalent to longitude) and Declination (equivalent to latitude). If you stand at a place on the earth which has latitude 50°, then the stars directly above you have a declination of 50°.

Unlike the stars, the Sun and Moon do not have a fixed declination. As the Earth travels its annual orbit around the Sun, with its rotational axis tilted at about 23.5° from the axis of orbital motion, the Sun's declination changes from +23.5° in June to -23.5° in December. Thus, in the Northern hemisphere, the Sun is higher in the sky in June, causing summer, than it is in December, when the Sun is low in the sky and is only above the horizon for a short time, causing winter.

The Moon also changes in declination, but it does so in only a month, instead of a year for the Sun. So it might go from a declination of +25° to -25° in just two weeks, returning to +25° two weeks later. Thus, in just one month the moon can move from being high in the sky, to low on the horizon, and back again.

But, unlike the Sun, the maximum and minimum declination reached by the Moon also varies. This is because the orbit of the Moon's rotation about the Earth is inclined by about 5° to the orbit of the Earth's rotation about the Sun, and so the maximum declination of the Moon varies from (23.5°-5°)=18.5° to (23.5°+5°)=28.5°. The effect of this is that at one particular time (the minor lunar standstill), the Moon will change its declination during the month from +18.5° to -18.5°, which is a total movement of 37°. This is not a particularly big change, and may not be very noticeable in the sky. However, 9.3 years later, during the major lunar standstill, the Moon will change its declination during the month from +28.5° to -28.5°, which is a total movement of 57°, and which is enough to take it from high in the sky to low on the horizon in just two weeks.

Detailed explanation

Finally! Why the Moon Looks Big at the Horizon and Smaller When Higher Up
Don McCready, Professor Emeritus
Psychology Department
University of Wisconsin-Whitewater
Whitewater, WI 53190

Revised November 10, 2004.

Nearly all people will agree that the picture at the right represents approximately how the horizon moon's size looks when compared with how it looks later, with the moon higher up in the sky.

If you wonder why this famous moon illusion occurs, you should read the following article, for, as illusion researchers know, a new explanation is needed: The explanations currently offered by textbooks and the popular media (including virtually all the 'moon illusion' sites on the internet)
simply do not explain the moon illusion.

This article reviews a new theory previously presented (since 1983) only in some technical articles in specialized publications for vision researchers (cognitive psychologists).
This article is long because, first of all, it describes the moon illusion more completely and in a more logical way than do conventional discussions.

Most moon illusion researchers now accept this new description.

Secondly, this article reviews the currently best-known theories, and shows why vision scientists don't accept them.

Thirdly, this article reviews the new theory that the moon illusion is an example of the less familiar, but ubiquitous, "size" illusion known as oculomotor micropsia/macropsia.

Finally, in order to complete the theory, this article reviews an explanation for oculomotor micropsia.

In other words, the new theory for the moon illusion is not simple: But it currently is the most satisfactory explanation.
Explore the Moon... discover its dramatic features and phenomena - often beautiful, sometimes bizarre, always changing. Inconstant Moon will take you on a new tour each night, with maps, photos, explanations, animations, selected links and even music!

The Moon is the most easily observable astronomical object, and also the most rewarding. For the beginner, it is a breathtaking spectacle through even a modest optical instrument, and as the knowledge and resources of the astronomer increase, it will continue to provide fascinating new challenges and insights. Inconstant Moon is intended as both an introduction to lunar astronomy for the beginner, and an ongoing reference point for the more experienced observer.
Inconstant Moon
The Moon at Perigee and Apogee

One of my favourite science fiction stories is Larry Niven's Inconstant Moon, about a night when the full Moon shone brighter than ever before. I won't say any more about the story so as not to spoil it for those who have yet to discover this most atypical gem in Niven's vast treasure chest. Find it; read it; enjoy!

Everybody notices the phases of the Moon, but to most people every full Moon is alike--the rising or setting Moon looks large due to perspective's playing tricks on the eye, but surely the full Moon high in the sky is always the same, right? Wrong.

One of the most spectacular phenomena in naked-eye astronomy escapes notice by the vast majority of people simply because the the eye and brain can't compare the size and brightness of objects observed on separate occasions.

This page explores the inconstant Moon in our everyday sky. While not as dramatic as that conjured up by the imagination of Larry Niven, we'll discover in it a celestial phenomenon seen by everybody, yet observed by only a few individuals.

Earth's Eccentric Companion

The Moon's orbit around the Earth is elliptical, with a substantial eccentricity (as major Solar System bodies go) of 5.49%. In addition, the tidal effect of the Sun's gravitational field increases the eccentricity when the orbit's major axis is aligned with the Sun-Earth vector or, in other words, the Moon is full or new.

The combined effects of orbital eccentricity and the Sun's tides result in a substantial difference in the apparent size and brightness of the Moon at perigee and apogee. Extreme values for perigee and apogee distance occur when perigee or apogee passage occurs close to new or full Moon, and long-term extremes are in the months near to Earth's perihelion passage (closest approach to the Sun, when the Sun's tidal effects are strongest) in the first few days of January.

The image above [see web site] shows how strikingly different the Moon appears at a full-Moon perigee and apogee. Most people don't notice the difference because they see the Moon in a sky that offers no reference by which angular extent may be judged. To observe the difference, you have to either make a scale to measure the Moon, or else photograph the Moon at perigee and apogee and compare the pictures, as I've done here.

The following table shows larger images of perigean and apogean full Moons, with details of the position of the Moon at the moment the pictures were taken. If your screen can't display the images one above another, use the side by side image above to appreciate the difference in size.
Inconstant Moon
by Larry Niven

What would you do if this were your last night on earth?


I was watching the news when the change came, like a flicker of motion at the corner of my eye. I turned toward the balcony window. Whatever it was, I was too late to catch it.

The moon was very bright tonight.

I saw that, and smiled, and turned back. Johnny Carson was just starting his monologue.

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.

Moon and Mars - Videos