q & a

Here are some of the questions which visitors to Inconstant Moon have asked. They have been divided into six sections...

movement: the moon's orbit and phases
physical: form, characteristics and geology
phenomena: unusual lunar events
observation: what can be seen... and what can't
terminology: naming of the moon, phases and features
miscellaneous: anything not covered avove

If you cannot find the answer to your question, go ahead and ask it!

first telescope

q: In a surprise request, my 8 year old daughter said she wanted a telescope for Christmas. A week or so later, in September, we experienced a very clear night on or near the full moon. I let the girls, 8 and 6 years old, look at the moon with a pair of auto-focus binoculars. It was not really that great of a view, but they got so "psyched" I had trouble getting them to fall asleep. Could you suggest what to look for in an appropriate first telescope that would keep in mind the family budget?
a: An excellent question - there are some traps out there for the newcomer, and it's very easy to buy the wrong instrument, have a disappointing experience and give up on astronomy altogether. With a little care, though, your daughters could be inspired to follow an interest which can last a whole lifetime!

A good beginner's telescope would be a simple, lightweight one with a good field of view and good quality optics. Decent telescopes start at around $250. If this is more than you want to pay, seriously consider getting a good pair of binoculars. You'll get much better results with a well-chosen pair of high quality binoculars than a "more powerful" telescope with poor optics.

Here is the most useful single piece of advice I can give: do not judge a telescope by its magnifying power. You will sometimes see small telescopes sold on the strength of enormous magnification. This is almost always a bad sign. The two things you really need from a telescope are the ability to collect lots of light and the ability to produce a detailed image. The key to both of these is aperture, ie having a decent-sized main lens or mirror (called the primary). Any telescope could have a million-times-magnification eyepiece attached, but without enough aperture the small amount of light collected would be diluted to the point of invisibility (for the same reason that even the very best torch could never light up a whole football field).

These are your basic choices -

  1. Refractor. This is the standard telescope which uses a series of lenses. On the plus side it produces a right-way-up image, so you could use it for bird watching too. On the minus side, for several technical reasons, good refractors are expensive to make. Compromising on price has a greater effect on quality with a refractor than with any other kind of instrument.
  2. Reflector (also called a Newtonian). This uses a curved mirror to do the magnifying, and the image is upside-down (not really a problem for astronomy as space has no "up"). They are easier to manufacture well, so you get a lot of aperture for your money. A Dobsonian (a special type of Newtonian) is particularly good value - $250 can buy you a 6" aperture. A quick tip for coping with upside-down images: if you want to nudge the telescope, say to follow something, just imagine you are pushing the image instead of the telescope itself.
  3. Catadioptric - this is a hybrid reflector/refractor, and falls between the two in terms of price. Maksutov-Cassegrain is the type most likely to fall into the beginner's budget, and if you can run to $400-500 you can get one with "Go To" electronics which will do all the finding and following for you. Some people enjoy the speed of "Go To" and not having to use star charts, others think using charts and "learning the skies" is all part of the fun. It's your choice.
  4. Binoculars. Many people (myself included) would not be without a good pair of binoculars even when using a larger instrument. Using both eyes gives a very immersing experience, and because they are hand-held you can find things very quickly. Good (for astronomy) means decent optics, large aperture, and not too much magnification. The ideal is 7x50 or possibly 10x50 - the figure before the x is the magnification (too large and the image becomes very shaky), and after the x is the primary lens diameter in mm (too small and the image becomes dim, too large and the binoculars become heavy to handle). Think of spending about $100+. A useful option is a camera tripod with a binocular adaptor.

The market leaders for telescopes are Meade and Celestron. Both produce excellent instruments. Astronomy magazine carries advertising from many reputable manufacturers and suppliers, and plenty of articles appropriate to a family starting in astronomy.

One final plea. If you buy a telescope from a responsible manufacturer this will not arise, but if the instrument you buy has a solar filter which fits to the eyepiece, THROW IT AWAY. Even with the aperture reduced, the glass in these filters becomes very hot and can explode, leaving the eye exposed to a powerful, focussed beam of sunlight. The consequences do not bear thinking about.

universal time

q: Can you explain Universal Time and how I calculate it in relation to the South of England? (Jody, Salisbury, UK)
a: Universal Time (UT) is what astronomers use when they describe events which happen simultaneously for all observers. For instance, anyone watching a lunar eclipse will see the Earth's shadow appear on the Moon at the same moment, wherever they are in the world. An astronomer might say that the total phase of the eclipse will begin at 15:23 UT or perhaps 15h 23.0m UT.

On the other hand, in an occultation the Moon may appear to pass in front of a star or planet at a range of different times depending on your location. These will often be expressed in local time.

UT is sometimes called Universal Time Co-ordinated or UTC. It has no time zones and it doesn't change during the summer. Observers in the UK are lucky: UT is the same as Greenwich Mean Time (GMT) and one hour different from British Summer Time (BST), so during the winter you don't need to change it and during the summer UT = BST - 1h.


q: In the film 2001: A Space Odyssey, in the crater Tycho was buried the Monolith which is activated by the sun. According to a friend of mine if one were to follow the "timeline" in 2001, on Thursday 26th April was the date the Monolith transmitted a signal.

I informed him that his "data" is incorrect, as the sun doesn't get close to Tycho until the 30th of April. Assuming the Monolith was buried in the centre of Tycho, what time would sunlight strike the Monolith? (John Goerger, Orange Ca)
a: You would both appear to be correct! This quote from an early script for the film would tie in with your friend's timeline: "The first surface was exposed at 0843 on
2001: A Space Odyssey
the 12th April... Let me see... that would have been forty-five minutes after Lunar sunset." However, you are entirely right in pointing out the error in these dates. Sunrise at the mid-point of Tycho is due at 12:00, 1 May 2001 UT (5:00, 30 Apr 2001 PDT).

These are not the only technical faults with 2001. In the relevant scene (shown here courtesy of MGM/UA) the Earth is seen in the lunar sky. It is far too close to the horizon, incorrectly suggesting that Tycho is close to the lunar pole. More relevantly, the Earth shows a gibbous phase. Sunrise at Tycho happens when the Moon seen from Earth is at gibbous phase, in which case the Earth seen from the Moon will be at crescent phase.

mr gorsky

q: When leaving the surface of the Moon, Neil Armstrong said goodnight to someone (ie "goodnight Mr Gurski"). What did he say, and why did he say it? (Fred Kolthay, Mahopac, NY)
a: A story has been circulating for some years on the Internet relating to the comment "Good luck, Mr Gorski" spoken by Neil Armstrong at the end of his first moonwalk. The meaning of these cryptic words is clever, risque and funny (and unfortunately cannot be explained on a family site). The only thing which detracts from it is that it never happened, as the mission transcripts reveal.

lunar holidays

q: Is it true that some people have paid to have a holiday on the Moon (some day)?
a: It seems unlikely that any bookings have been taken, as timescales and prices are an unknown quantity, but several large hotel and catering (including Hilton, Sheraton and McDonalds) have begun making serious plans. Architects have devised innovative construction techniques, and food technologists have been working on new methods to grow, manufacture and cook food. The greatest limiting factor is the exorbitant cost of transporting each pound of material from the Earth. Any long-term lunar population needs to be largely self-sufficient.

the moon an artefact?

q: Has it ever occurred to you that the Moon was put in place by aliens? The Moon doesn't rotate, very unusual. It is the exact distance from the Earth to obscure the Sun during eclipses. It seems to be hollow as the result of seismic testing by our astronauts. Bright lights and unusual structures have been seen on the Moon since there have been telescopes. Finally, the Earth and Moon are different composition and ages, the Moon isn't part of the Earth. (Tom Anger, Houston)
a: There is a scientific/philosophical principle called Occam's Razor which says, in essence, that when seeking an explanation you should not make any more assumptions than are necessary . Bearing that in mind, consider the points one at a time...

  1. The Moon does rotate, but at the same speed as it revolves around the Earth so we always see the same face. It's called captured rotation, and it's not unusual - pretty much every satellite in the solar system does it.
  2. Seismic testing shows the Moon to have a low density with a small iron core. Many objects in the solar system have much lower densities still. Amusingly, its shockwave propagation is similar to that of cheese!
  3. There are indeed many reported observations of Transient Lunar Phenomena (TLPs) - temporary bright or fuzzy patches on the Moon's surface. These are generally thought to be the result of outgassing. The Moon may be geologically extremely quiet, but not quite dead.
  4. The Moon does have a different composition to the Earth, but with much in common too. It is thought to have been formed by the grazing impact of a Mars-sized object with the early Earth - much material was exchanged, but the proportions are different and some minerals are unique to the Moon. Both are about 4.6 billion years old, but geological activity means little of the Earth's original crust remains.
  5. The Sun is 400 times as large as the Moon, and around 400 times as distant. They therefore appear just the same size, which is why solar eclipses are possible. There can be no denying that this is a unique and amazing phenomenon, but it would be a strange world with no coincidences!
The bottom line is this: whilst perfectly reasonable explanations exist for these observations, there is no compelling reason to introduce the radical notion that the Moon is an artefact.