When did the Moon become oval instead of round?

November 27, 2015 10:15 PM

This evening, as I was making my way home, I looked up through my glasses and noticed that the Moon was not round, but oval in shape. However, when I squinted (which brings things into focus), the Moon became round. Now I don't know if my eyes have gotten much worse in the last two years, or if the prescription I was given was wrong.

Two years ago, I decided to get a new pair of glasses, and a new prescription for them. The shop where I was getting the frames that I liked did not, however, perform eye tests, so I went to an eyeglass shop in my neighborhood where I just had the test done (but did not buy any frames there). I had the sense that they did not appreciate the fact that I was having them do the test, but not buying the frames/lenses from them.

Anyway, when I finally got my new glasses, I noticed that while they made distant things clearer, close things were blurrier than they were without glasses on (more so than seemed to be the case with my previous 10+ year old prescription). When I went back to the shop where I bought the frames and lenses and mentioned this to them, they more or less said it was because of my aging eyes, and that if at my age I wanted both distant and near things to be in focus I would need bifocals.

Frankly, I can't remember if the Moon has looked round to me on previous occasions wearing these glasses. But I have wondered since that eye test if the prescription I got was accurate. There were many points during the eye exam at which I really did not know what to answer, and it didn't help that I felt the man giving the test was displeased that I was just getting the test there and not buying their frames.

How likely is it that my prescription has been, somehow, slightly off from the get-go (due to the factors mentioned above), and how can I determine if the test was poorly done vs. me just expecting too much of my old eyes and the correct prescription that's doing the best any prescription can do at this point to help them see clearly?

posted by tenderly to Health & Fitness (8 answers total)

It is possible just in general to get a bum prescription, but you should also usually be going more like annually, and certainly more than once every ten years. It's very possible that in the course of two years, your prescription has changed enough that you need something different, and if you're older, yes, that's probably going to mean you need bifocals. I'm in my 30s and was struggling the last couple months and got to the doctor to discover that for the first time, I have an astigmatism in one eye, and that while I don't question at all that I needed a prism lens before, that seems at some point to have cleared up. And that's just since, like, last year.

If you don't want bifocals, you might look into a separate pair of reading glasses, that's what my mother used for years since she didn't like either the old-fashioned or the progressive lenses; she just switched glasses for reading or the computer.

The nice thing is that you should be able to, if your frames are in good shape, just get new lenses put in the frames you already have, so maybe this time you can go ahead and get your lenses from the same place doing the exam, which should make it a bit easier to work out the details. I've gotten glasses online from Zenni Optical and stuff before, but I always get the first pair locally to make sure everything's okay.
posted by Sequence at 10:55 PM on November 27, 2015 [1 favorite]

I am not entirely sure how you expect us to give a reasoned opinion about an eye exam two years ago when we have no idea about your general eye health. That said, I will give you three thoughts:

1) The moon is not actually round.
2) It is common for astigmatism correction on glasses (which you may or may not have) to distort your perception of shapes - astigmatism correcting glasses turn rectangles into trapezoids. You tend to get used to it rather quickly but if you stare enough you can often notice the effect.
3) A prescription from two years ago is about a year out of date - go get a new exam.
posted by saeculorum at 10:57 PM on November 27, 2015

it's certainly true that, as you get older, things nearer become more blurred. i now have vari-focals, which help (but are not perfect by any means).

also, you can get a bad prescription. i once got a prescription that was so different to a previous measurement that, when i realised, i went back and questioned it. it turned out the optician had made a mistake writing down the numbers.

finally, i don't think you can infer much about a prescription from how things look out of focus. i am not an optician, but i was an astronomer / physicist and i can't think how you can make an argument like that. first, i don't think out-of-focus images are included in how the lens is designed. second, it seems like, unfocussed, you are going to see something related to whatever they are intended to correct for (so the comment about astigmatism above seems to be on the money).

so in summary: you may be right that the prescription is wrong, but the reasons you give don't support that, and my vote would be for old age. and i don't see any way you can argue that it is anyone's fault (to the extent that you get it fixed or money back) after two years.
posted by andrewcooke at 1:56 AM on November 28, 2015 [1 favorite]

The moon today is in its waning gibbous phase. That means it actually looks oval right now, not round.
posted by The Toad at 3:20 AM on November 28, 2015 [10 favorites]

Speaking as someone with completely dysfunctional eyes: putting the moon's actual shape aside, one would expect a two-year-old prescription to be less-than-useful at this point, whether in one eye or both. You need an annual exam.
posted by thomas j wise at 5:16 AM on November 28, 2015 [1 favorite]

I have an astigmatism, and for at least 20 years, my prescription changed every 2-3 years, or as often as I went in for an exam, anyway. I once broke my glasses and had to wear a 5-year-old pair of backups, and tripped over everything for 2 days until I could get them fixed because the ground was about an inch closer than it appeared in the old glasses.

Now that I'm older, it's the reading portion of my progressive lenses that changes, but still, pretty much every 2-3 years I need new lenses, so your situation doesn't surprise me.
posted by Devils Rancher at 5:43 AM on November 28, 2015

The moon is, in reality, incredibly close to round - it's round to one part in 900, which is imperceptible to your eyes. There are lots of ways that the Moon can appear non-round from Earth that have nothing to do with your glasses, however. The aforementioned fact that the Moon is in its gibbous phase is one. The other possibility is atmospheric diffraction. In particular, when the Moon is very close to the horizon, differential refraction may distort the Moon's shape.
posted by Betelgeuse at 6:21 AM on November 28, 2015 [2 favorites]

If I had to vote, I would guess that your prescription changed enough that the spatial distortion in your lenses -- which is normal and AFAIK unavoidable -- became a little stronger so you notice it. Look at a window or something rectangular and move your head so you're looking through different parts of your lenses and you'll see its shape change.
posted by ROU_Xenophobe at 9:34 AM on November 28, 2015