Cosmic Quiz – Why is the moon sometimes visible during the day?

Seth Jarvis

This week 6-year-old Kambreigh Schooley asked us why she can sometimes see the moon during the daytime…What a great question!

To answer this question you need to first remember that one half of the moon is always illuminated by the Sun.

As the Moon travels around Earth in its 29 day orbit, the portion of the illuminated half of the moon that is visible to us is always changing, creating the different moon phases as we see in the diagram below.

Lunar Phase Diagram. Image courtesy www.wikipedia.com

Phases of the Moon. Click to enlarge. Image courtesy www.wikipedia.com

Moonlight is light from the sun bouncing off of the moon and reflecting back to us on Earth.  It’s very bright!  Full moons are the brightest moons because virtually all of the illuminated portion of the moon is visible to everyone on the night-time side of Earth.  First Quarter and Third Quarter moons only show us half of the sunlight side of the moon.

Because nighttime moonlight is so bright and beautiful it’s easy to overlook the fact that moonlight is also bright enough to be easily seen against the blue sky during the day.

Moon as it appears in the daytime sky.

Moon as it appears in the daytime sky. Click on image to enlarge

Now back to the question about why the moon is sometimes visible above the horizon during the daytime hours.  Take another look at the moon phase chart above.  Imagine yourself standing at the various locations on Earth indicated by the time of day.

You can see that at the 12 o’clock noon position, you’re directly under the Sun, and if you happen to be looking up on the day of the New Moon, you’ll not only be looking toward the Sun, you’ll also be looking in the direction of the dark half of the moon.  Not much fun there.

Now imagine yourself at that noon location on Earth, but the moon’s phase is either First Quarter or Third Quarter.  Can you see the moon?  Sure!  Your line of sight permits you to see the moon quite well, even though it’s daylight.

It’s generally been my experience that the best time to try daylight observations of the moon is in the week following Full Moon.  Look in the west or southwestern skies during the mid-morning hours. If the skies are clear, you’ll have no trouble seeing the moon, in broad daylight.

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20 thoughts on “Cosmic Quiz – Why is the moon sometimes visible during the day?

  1. Busted! You got me, Tyler.

    Yes, the moon makes a full 360-degree trip around the Earth (its “orbital period”) in 27 days 7 hours and 43 minutes.

    However, the time interval between Full Moons, the measure of how long it takes for us to perceive that the moon has made a full circuit around the Earth, is 29 days 12 hours 44 minutes. That’s known as the “synodic period” of the moon.

    If we tried measuring the moon’s motion with its orbital period the phase of the moon would never match from one orbit to the next. By using the synodic period it is possible to make uniform statements about the “age” of the moon during each lunar cycle.

    But nonetheless, you got me, and it was technically inaccurate to refer to the moon’s “orbit” as being 29 days long.

    Thanks for keeping us honest!

  2. Wow this is sooooooo cooooool and i dont cosider myself a nerd nerd (no offense) but when it comes to space i lose my coolness

  3. The “Cosmic Quiz” questions are solicited through a partnership with a local tv station from viewers. Selected questions are answered by a variety of staff members, including our Director and our Education staff. We love answering questions from the public about astronomy and space science, and would spend more time doing it if we could.

  4. hi im only thirteen and im doing this kind of project, do you know what my activity will be? plz help me i cant think of any activity about “moon in the morning”

  5. We are not in the habit of doing students’ school work for them, but if you give us more information about the nature of your project, we can probably give you some ideas on how to get started.

  6. dont worry i did all of my work i just need an activity (science expirement) about moon in the morning…………….. i was actually thiking of a flash light presenting a sun, a mirror present a moon, and a light paper presenting a the sky.

  7. I know that you can occasionally see the moon during daylight hours, but my question is can one see a FULL moon during daylight hours? I am asking this because one of my teachers told me that you can’t because the earth, moon, and sun are aligned at 180 degrees which would prevent us here on earth from viewing a full moon while the sun is up. Is this true? I feel like I’ve seen it before and think that this 180 degree thing doesn’t hold true because earth is tilted 23.5 degrees.

  8. @Erin:

    Your teacher is mainly correct. However, if the moon were 180 degrees away from the sun, it would be in Earth’s shadow (a Lunar Eclipse). A Full Moon occurs when the Sun/Moon angular separation is close to 180 degrees, but not exactly (say 176 degrees). Ignoring atmospheric refraction, if you were in the right place, you could actually see both the sun and Full Moon at the same time, but they would both be on exact opposite sides of the sky and just above the horizon (example: sun due east, 2 degrees alt, moon due west, 2 degrees alt).

    Earth’s 23.5 degree tilt effets where the Full Moon will be at in the sky, but it does not change the illumination of the moon’s disk. Angular separation is angular separation, and Earth’s own axial tilt has nothing to do with it. If the sun and moon are separated by 135 degrees (gibbous moon), and you take into account Earth’t tilt, you would only notice a change in the moon’s location in the sky. It’s percentage of illumination would still be the same, as the two bodies are still separated by 135 degrees.

    What you’ve probably seen is a nearly-full gibbous moon, just a couple of days before or after the Full Moon. The gibbous moon is visible either a couple of hours before sunset or a couple of hours after sunrise, and at first glance appears to be full. Upon closer examination, you will see the terminator clearly (the divider between the day and night hemispheres).

  9. This morning, I saw the moon during daylight hours in a way that I never have before. It showed above the mountains and the moon was as bright and as solid appearing as it looks at night. I know that this full moon was a couple of days ago. I have never had any trouble being able to see that the moon isn’t really full if I look carefully. But I couldn’t see that this morning. The moon looked full and as vibrant as it does on a clear night. Very different experience that I will not forget.

  10. Why do I see the moon in the west just before night fall (7/3/2011, 9:00pm Texas)

  11. Vicky,

    Refer to the image at the top of the post.

    When you’re looking towards the Moon near sunset (sometime between 6pm and 9pm) soon after the New Moon, you’re looking almost as much in the direction of the Sun as you are in the Moon. With each passing day the Moon will move a little more to the east until on the day of the Full Moon the Moon rises in the east as the Sun sets in the west.

    The Moon keeps moving around Earth in an easterly direction until the next New Moon and the cycle begins again.

  12. ok ive seen the moon in the sky before but ive pointed it out to my friends and they all say that they can’t see it. I know i saw the moon.

  13. Ok I have had this question for a while now and no one has been able to answer it and I can’t seem to find the answer when I search google but I live on the south east coast of the USA and I saw the sun and the moon today in the daytime it was around 3:00pm and I was wondering if we have both the sun and the moon at the same time what does the other side of the world have?

  14. Hello Heather,

    People elsewhere in the world who are east of you (e.g. Europe) have already seen the Sun and Moon in the sky the same as you, and people west of you (e.g. Hawaii) will be seeing the Sun and Moon together in the sky when those object rise in their sky just as what you’re seeing rises into your view. It’s just a matter of longitude and time zones.

    Put a small earth globe on a table and stick a bit of tape on your location. Then put a tape marker onto locations in Europe and Hawaii. Now place something on the other side of the room that can represent your “Sun.” Slowly rotate the Earth globe counterclockwise (as viewed from above the North Pole and looking down) and imagine your view of your “Sun” from each of the positions you’ve marked on your globe as the world’s turning brings a view of your Sun first to your easter horizon (sunrise), then through noon, and on to the Sun setting as you rotate yourself eastward causing the Sun to disappear against your western horizon (sunset).

    The Moon orbits the Earth at a distance of 30 times Earth’s diameter, so it can be anywhere in a circle of that radius around the Earth on a plane that is fairly close to the line drawn between the Earth and the Sun. You’ll soon see how it’s possible for both the Moon and Sun to be visible in the afternoon sky, or in the mid-morning sky, or only the Moon visible, or no Moon visible at all. It’s all about the constantly changing angles between the Sun and the Moon as the Moon orbits the Earth and the Earth spins on its axis.

    I hope that helps.

  15. I think Heather’s question was ‘what does the other side of earth see at the same time that we are seeing the sun and part of moon?’ Since it’s night there. Also translating to the question “do we see opposite phases of moon during day and night?” The answer to my understanding is yes, we see opposite phases of moon during day and night. Please correct me if I’m wrong.

  16. Which means the other side of world is seeing the other half of full moon. And we will see that half of moon in the night. That’d explain solar eclipse on the day of no moon in night.

  17. What does the other side of the Earth see?

    Whatever is 180 degrees in the other direction. If the Full Moon is high above Utah, it’ll be morning in Paris, midday in Karachi, and late afternoon in Japan. Then as Earth rotates, what was above Utah becomes the sky above Hawaii. Think of riders on a merry-go-round looking at a building in the distance. Sometimes you can’t see the building because the merry-go-round is in the way, but as you rotate around you find that you can see the building… until your motion carries you again out of view. People elsewhere on Earth on the same north-south hemisphere see the exact same sky that we do, just a few hours later or earlier than we do, depending on their longitude. As latitude changes the view of the sky you have also changes, because you’re now changing what portion of Earth is blocking your view of the heavens not due to rotation, but the fact that the world is simply there. An ant on a basketball’s “north pole” can’t see what an ant on the ball’s “south pole” sees.

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