PDA

View Full Version : Moon phase

jdh
2013-Aug-23, 04:32 AM
I had a homework problem in astronomy class that said

The __________ moon is visible above the western horizon a couple of hours before sunrise.
The correct answer is supposed to be "waning gibbous". It seems to me that it should be waxing gibbous. What am I doing wrong?

By coincidence, the day before yesterday, August 20, the moon set at about 6 am here, and the sun rose at about 6:30, so two hours before sunrise the moon was just above the western horizon, but it was not full until 6:45 pm, which means that it was waxing gibbous, right?

She says it was waning because the angle is between 180 and 240 degrees, while waxing gibbous is between 90 and 180 degrees, but I can't find in the book where it says which direction to measure from. I'm sure that my teacher must be right, but I cannot see what I'm doing wrong.

grapes
2013-Aug-23, 09:36 AM
Interesting question!

According to this site ( http://www.MoonPhases.info/full_moon_calendar_dates.html ) the moon was full at 3:45 am GMT Aug 21, which would have been 8:45 pm PDT on the day before, Aug 20. But I do see other almanac sites saying the full moon was at 6:45 pm PDT on Aug 20. So, it was full on Aug 21, Greenwich Mean Time, but full on Aug 20, Pacific Daylight time.

Times of the setting moon are affected by the geometry of latitude--an observer at the north pole in winter might not see the full moon set at all. So, it looks like you're right, local geometry can slightly influence the actual timing.

However, on Aug 20, this site ( http://www.timeanddate.com/worldclock/astronomy.html?n=224&month=8&year=2013&obj=moon&afl=-11&day=1 ) says the moon set at 6:06am PDT in SF and the sun rose at 6:30am PDT. It says the moon set at 7:16am on Aug 21.

Robert Tulip
2013-Aug-23, 09:39 AM
Best answer is full moon, which sets in the west about dawn, so is visible above the western horizon before dawn. Waxing gibbous moon sets before dawn. Waning gibbous - post full - sets after dawn, so the moon is visible in the western sky in the wee small hours for several days before and after full.

grapes
2013-Aug-23, 09:55 AM
Best answer is full moon, which sets in the west about dawn, so is visible above the western horizon before dawn. Waxing gibbous moon sets before dawn. Waning gibbous - post full - sets after dawn, so the moon is visible in the western sky in the wee small hours for several days before and after full.
So, the answer to the quiz question in the OP is both? It could be waning gibbous, or waxing gibbous?

ETA: Does "above the western horizon" mean just above, or could it also mean high above?

antoniseb
2013-Aug-23, 12:46 PM
I think the answer depends on how far from the Western Horizon you can let it be. If it is exactly on the Western Horizon a couple hours before Sunrise then yes, it should be Waxing Gibbous. Waning Gibbous at that time should be somewhere between due South and the Western Horizon (assuming you're in the Northern Hemisphere). I think the author of your homework problem made an error.

Jeff Root
2013-Aug-23, 02:07 PM
I saw the Moon in the west the other morning. Looked full.

-- Jeff, in Minneapolis

grapes
2013-Aug-23, 02:11 PM
It was full, at some point. A blue moon, old-style.

jdh
2013-Aug-23, 05:57 PM
I wrote the wrong date, I should have written the 20th. Also, I mixed up standard and daylight saving time. I have fixed this in the original post.

It makes little difference. According to USNO (http://aa.usno.navy.mil/data/docs/RS_OneYear.php), on August 20 the moon set at 05:06 PST. The moon was full at 0145 UTC, or 5:45 PM PST. The sun rose at 5:30 PST, so a couple of hours before sunrise was 3:30, and the moon was well above the western horizon.

On August 19, the moon set at 3:55, which means that at 3:30 it was just above the horizon, and more than a day before the full moon, which were the conditions of the test question.

jdh
2013-Aug-23, 10:32 PM
OK, she is saying that the moon is waning gibbous because the angle is between 180 and 270 degrees, and to be waxing it must be less than 180. However, in searching the Internet I read various places that elongation is measured to the east from the sun. She seems to be measuring to the west.

With the sun below the eastern horizon, it seems to me that you have to measure downward, past the nadir, then up towards the moon in the west. By my calculation, the eastward elongation of the moon from the sun would be around 150 degrees, which according to her own words makes it waxing gibbous.

jdh
2013-Aug-24, 12:11 AM
She just posted this reply:
Angles are usually measured counter-clockwise but regardless of that, here is the easiest way to understand this:

1. At full moon, the moon rises when the sun sets and sets when the sun rises.

2. That is 180 degrees between the two.

3. So, on full moon day, when the Sun is a couple of hours below the horizon, the moon will be ablout 30 deg. above the western horizon.

4. The very next day is the start of the waning gibbous phase.

5. This phase lasts approximately 7 days and the moon covers about 90 deg. in its orbit. So the moon moves about 14 deg. per day.

6. So, the next day after full moon, when the sun is again a couple of hours below the horizon, the moon will be about 15 deg. above the western horizon.

7. The day after that it may still be above the western horizon but just grazing the surface.

8. The waxing gibbous moon on the other hand, will be well below the horizon, at the same time because it lags behind the sun by less than 180 deg. It won't be just above the western horizon.

This is the easiest way to visualize it. Or, make a model with a bulb and a styrofoam ball with the east on your left and the west on your right and convince yourself.

Rather than figuring out where you are going wrong in your reasoning, I think this is a faster way.

Actual timings may be misleading because the sun spends a lot more time over the horizon during spring and summer than at other times.
Is what she said correct?

Hornblower
2013-Aug-24, 12:56 AM
Good grief! She has the motion backward on the days following full Moon. On successive days the waning gibbous Moon will be higher in the western sky two hours before sunrise. A waxing gibbous just before full could be just above the horizon, but may be indistinguishable from perfectly round.

Try to find a way to call her on her error, politely of course.

jdh
2013-Aug-24, 01:11 AM
Since she told me to visualize it, I made some diagrams. However, when I went to upload them, I saw that her last post has disappeared! I think that the wheels are turning...

jdh
2013-Aug-26, 02:40 AM
She said I am right, but now is blaming the testing software. GIGO

Jens
2013-Aug-26, 02:45 AM
She said I am right, but now is blaming the testing software. GIGO

I think she could just admit she was wrong and leave it at that. I teach English from time to time, and it would be a big lie to say I know everything and never make a mistake. . .

Amber Robot
2013-Aug-26, 07:43 PM
You could have just put "Earth's" and been completely correct. :p