Keeping a Sky Log: What to Write Down, How Long to Keep Going and What the Notes Show

A sky log is a notebook where you write down what you see overhead, with the date and time beside it. It sounds small, but a few weeks of entries show things that a single look never does. The Moon, for example, rises a little later each day, and a log is the easiest way to see that for yourself.

This guide follows what NASA and the education team at its Jet Propulsion Laboratory (JPL) ask journal keepers to write down. It covers how long to keep going, how to read your entries and how to check them against predictions. It does not invent a form of its own. It ends with a task in the Rise & Set Dial.

Why keep a sky log

NASA's Moon Observation Journal opens with a simple invitation: spend the next month getting to know the Moon. JPL explains why that works. The Moon is the most visible object in our night sky, and it is even sometimes visible in daylight. Keeping a Moon journal for a month, JPL says, is a great way to see the patterns in its phases and to predict the phase that will appear the next night and the next week.

A log does not have to be serious to be useful. NASA's skywatching FAQ says skywatching can be as casual as a couple of minutes of stargazing, and it counts noticing the changing position and phases of the Moon over a few days. JPL's lesson ties the activity to a school science standard about using observations of the sun, moon and stars to describe patterns that can be predicted. A few lines a night are enough to start.

What official journals ask you to write down

Several NASA and JPL pages describe journals. Their requests are short, and they overlap.

So the official models ask for a date, a time and a drawing. When you want to go further, they add the Moon's place in the sky and its rise and set times. This guide adds only one suggestion of its own, about writing down your place, in a later section.

For the Moon's place in the sky, NASA's skywatching FAQ offers a yardstick. Hold your hand out at arm's length. The width of your little finger is about one degree. Three middle fingers are about 5 degrees, and a fist is about 10 degrees.

How long to keep going

NASA's journal asks you to observe the Moon each day for a month. JPL says you may start on any day of any month, and to keep going every day for an entire month, at least 30 days. JPL's lesson adds that the activity can start during any part of the phase cycle.

A month matters because of the cycle's length. NASA says the cycle of phases repeats about once a month, every 29.5 days, and the U.S. Naval Observatory (USNO) gives the same average. A full cycle starts with a new moon, passes through seven more distinct phases and returns to the new moon, according to JPL. Thirty days covers the whole round.

NASA's Skywatching FAQ gives the orbit as about 27.5 days, so two NASA pages differ by a fraction of a day on that number. The 29.5 days is the one that matters for phases.

Missed nights need a plan, and the two sources give different ones. NASA's journal says that if you cannot see the Moon at all on a day, no matter when you look, you should note that in your journal and write down why. JPL says to leave that day blank and fill it in later: once you see the pattern, use it to predict what the Moon looked like and draw it in. NASA's questions even ask whether you could still work out what the Moon would have looked like. Either way, the gap is part of the record. JPL's lesson also warns that weather can interfere, and it advises against asking students to keep a log in an especially cloudy month.

Reading your log: the rising time

The most useful column in a log is the time. NASA's skywatching FAQ says the Moon rises, on average, about 50 minutes later each day during its month-long cycle. The reason is that the Moon is moving in its orbit around Earth. Each day Earth completes a full rotation, but the Moon has moved a little, so it does not come into view above the horizon until Earth turns a little further.

NOAA states the same number from another angle. A lunar day, the time for a place on Earth to turn from one point under the Moon to the same point again, is 24 hours and 50 minutes. The Moon revolves in the same direction that Earth rotates, so Earth needs an extra 50 minutes to catch up to it. NASA adds that we see the Moon shift about 12 to 13 degrees each night toward the east against the fixed stars.

That gives your log a pattern to find. NASA's journal asks whether you saw the Moon at the same time each day and whether there was a pattern to the times. There is one. The Full Moon rises at sunset, according to NASA, and each later night adds about 50 minutes. The waning gibbous Moon rises at night after sunset, and the third quarter Moon rises around the middle of the night. Because 50 minutes is an average, real nights vary a little.

Checking your log against predictions

A log is more fun when you can test it. NASA's phase page says its rise and set patterns hold at most latitudes and give only a rough estimate. For exact moonrise and moonset times, it says to check local predictions. NASA's Observe the Moon at Home page points to its Daily Moon Guide to check your work, where you can enter any date of the year to see the Moon's phase.

USNO offers a data service called Complete Sun and Moon Data for One Day. It provides rise, set and transit times for the Sun and Moon, the start and end of civil twilight, and the phase of the Moon for any year from 1700 to 2100. You give it a date, a place (a U.S. city or town, or coordinates) and a time zone, and the times come back on a 24-hour clock. The sunrise and sunset guide explains why even a good prediction can differ from what you see by a minute or more.

After a month, JPL suggests checking that your drawings are in the same order as the animation of the phases that it links to. NASA's journal asks you to find where your Moon most closely matched each of the eight phases, and to say how you think it will change over the next week.

Here is the suggestion promised earlier. None of the official sheets above has a field for your own place. Because NASA says exact rise and set times must be local, writing the town or the spot next to your entries makes later checking easier.

Common mistakes

• Writing only the date. NASA's journal asks for the time too, and the time is what shows the pattern of later rises.
• Filling a missed night with a guess right away. JPL says to leave the day blank and fill it in once you see the pattern. NASA says to note that you could not see the Moon, and why.
• Stopping after a week. JPL says at least 30 days, and the phase cycle takes 29.5 days on average.
• Expecting the same gap every night. NASA's 50 minutes is an average over the month-long cycle.
• Comparing your times with another town's. NASA says exact moonrise and moonset times come from local predictions.

Try it in the instrument

Open the Rise & Set Dial and match this log entry: the Moon rose at about 9 PM. The target is a Moon age of about 18.5 days. USNO counts the Moon's age in days from New Moon.

Here is the relation. The dial models a middle-latitude place near an equinox, with sunset around 6 PM. NASA and USNO both say the Full Moon rises about the time of sunset, and the Full Moon is half a cycle old, about 14.8 days. From 6 PM to 9 PM is three hours, or 180 minutes. At about 50 minutes later each day, that is roughly 3.6 days after Full Moon, and 14.8 plus 3.6 is about 18.4 days. The dial's model puts moonrise at 6 hours plus 0.81 hours for each day of age, which gives 9 PM at 18.5 days.

The answer also fits NASA's rise pattern. A moon that rises at night after sunset is a waning gibbous. Treat the arithmetic as a rule of thumb, since the 50 minutes is an average, and let the dial's window forgive a small difference.

Questions people ask

Do I need special equipment to keep a sky log?

No. JPL's project lists a printed journal page and a pencil, and its lesson says a plain sheet of paper works for drawings and notes. NASA's skywatching FAQ says you do not need any special equipment to get started skywatching.

Can I log the Moon in the daytime?

Yes. JPL notes the Moon is sometimes visible in daylight, and its observation sheet has a circle for day or night. NASA's rise and set list shows that some phases are up in the daytime sky, such as a waxing crescent that rises in the morning and a first quarter Moon that rises around midday.

How do I describe where the Moon is in the sky?

Use the yardsticks from NASA's skywatching FAQ: about 1 degree for your little finger, 5 degrees for three middle fingers and 10 degrees for a fist, all at arm's length. JPL's lesson asks observers where in the sky the Moon is located, so writing a direction and a height is in the spirit of the official journals.

Can I log planets and stars too?

NASA's skywatching FAQ counts all of these as skywatching: watching the Moon's changing position and phases, searching for Jupiter or Venus in the twilight sky, and noticing when a constellation such as Orion is visible for the first time in a few months. The journals above are written for the Moon, but the same habit of a date, a time and a note about where you looked works for anything you watch.

Sources

• Moon Observation Journal, NASA Science. https://science.nasa.gov/resource/moon-observation-journal. Opened November 10, 2026.
• Moon Observation Journal (printable PDF), NASA Science. https://assets.science.nasa.gov/content/dam/science/psd/earths-moon/outreach-materials/Moon%20Observation%20Journal.pdf. Opened November 10, 2026.
• Look at the Moon! Journaling Project, NASA JPL Education. https://www.jpl.nasa.gov/edu/resources/project/look-at-the-moon-journaling-project/. Opened November 10, 2026.
• Observing the Moon, NASA JPL Education. https://www.jpl.nasa.gov/edu/resources/lesson-plan/observing-the-moon/. Opened November 10, 2026.
• Observe the Moon at Home, NASA Science. https://science.nasa.gov/moon/observe-the-moon-night/observe-at-home/. Opened November 10, 2026.
• Skywatching FAQ, NASA Science. https://science.nasa.gov/skywatching/faq/. Opened November 10, 2026.
• Moon Phases, NASA Science. https://science.nasa.gov/moon/moon-phases/. Opened November 10, 2026.
• How to Find Good Places to Stargaze, NASA Science. https://science.nasa.gov/solar-system/skywatching/how-to-find-good-places-to-stargaze/. Opened November 10, 2026.
• Frequency of Tides - The Lunar Day, NOAA National Ocean Service Education. https://oceanservice.noaa.gov/education/tutorial_tides/tides05_lunarday.html. Opened November 10, 2026.
• Complete Sun and Moon Data for One Day, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/data/RS_OneDay. Opened November 10, 2026.
• Phases of the Moon and Percent of the Moon Illuminated, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/moon_phases. Opened November 10, 2026.

We opened each source ourselves on the date shown. The drawings are our own and were checked against the numbers in these sources.

Read next

• When the Moon RisesThe Moon rises about 50 minutes later each day, so over one cycle of phases moonrise slides around the whole clock.
• When the Moon SetsA Moon sets roughly half a day after it rises, and each day it sets about 50 minutes later.