Astronomers love a challenge, and one of the easiest to take part in is seeing the thinnest crescent moon possible.
We can only see the thinnest sliver of the moon when it is either very old or very young, i.e. just a few hours before or after the moment of new.
There is some planning involved to improve your chances of setting a personal record and, in this article, we’ll take a look at how to do it.
What is the Youngest Moon You Can See?
With one special exception, which we’ll come back to, it’s been shown that it is impossible to see the moon when it is less than 7.5° away from the sun, this is known as the Danjon Limit.
The Danjon limit makes it practically impossible to see a moon that is less than 12 hours old, but it is not so hard to see one that is 24 hours old, given the right conditions.
Record Young Moons
There are few records for new moon sightings, one is unbreakable but you might be tempted to try your hand at the other two.
The youngest moon ever captured was on July 8, 2013, by Thierry Legault. His image shows a crescent at the exact moment of new.
To achieve this unbeatable feat, he captured the image using near-infrared wavelengths and used image stacking and processing software to make the thinnest of crescents appear.
The record For a new moon sighting with the naked eye was captured by astronomer Stephen James O’Meara in May 1990, his crescent was just over 15 ½ hours old. That’s still too young to find any of the Lunar 100!
Add an optical aid and the record drops to just over 11 ½ hours old. This one was captured by Mohsen G. Miraseed in September 2002. He used huge binoculars and stood on a mountain to maximize his opportunity.
When Can We See A New Moon?
There is one relatively rare exception to the records above when we can see a new moon with the naked eye: during a total solar eclipse. At that time, the sun is directly behind the moon and the black disc we see is the moon itself.
Other than that, we can’t see a moon younger than 12 hours old either with our eyes directly or with binoculars or telescopes. It’s also very risky to try and see the moon that young because the blinding sun is so close by.
Young rooms are always captured in the day because what makes a moon new is being very close to the sun from our perspective. The moon is between us and the sun.
When to See a Thin Crescent Moon
Just like the Planets, the moon follows the line of the ecliptic.
The steepness of the ecliptic and its height above the horizon change with the seasons.
In winter and spring, the ecliptic is steeper and higher in evenings above the western horizon than at any other time of year. The opposite is true for early morning moon observers, i.e., try spotting the last few hours of the old waxing crescent moon shortly before sunrise in summer and fall.
The Moon is bigger than you think, but the best time to spot the thinnest moon you’ve ever seen is when the ecliptic rises steeply above the horizon because the moon will be riding high in the sky even though it is close to the sun.
Observers near the tropics have even better chances of seeing a tiny sliver or our neighbor because the ecliptic rises the highest and steepest there. Astronomers in Florida, for example, have a much better opportunity to spot a young moon than observers in Maine.
The most obvious time to try and set your personal record is when the moon is nearly new. This happens once every lunar cycle which is 29.5 days.
We get two opportunities to see the slimmed-down moon.
The first comes to us just before dawn on the day before the new moon. The slim crescent will rise shortly before the sun on that day.
Our second opportunity appears on the day after the new moon when we might catch a glimpse of its crescent shortly after sunset.
As we saw earlier in the article, sunset opportunities are best in winter and early spring whereas sunrise viewing is better in late summer and early fall.
How To Spy a Thin Lunar Crescent
When observing the thin old moon before sunrise, it’s easier to see it when you live further east; the opposite is true for observers hoping to catch a micro-thin evening crescent the day after new.
On the days immediately before and after the new moon, the lunar surface will be illuminated somewhere between 1% and 3%, depending on the exact moment of it turned new. An example of a 3% illuminated moon is given below.
One final element which needs to be in your favor to catch an hours-old moon is the viewing conditions.
As well as a cloudless sky, you should be able to see the horizon. For an evening hunt, you’ll need to see the western horizon. Morning crescent-chasers need a clear eastern horizon.
Where to Look for a Thin Moon
As mentioned above, you need to be looking towards the eastern or western horizons depending on the time of day you are hunting.
More specifically, it helps to pre-plan where exactly to look because you’ll only have a very short window of opportunity. The sun rises soon after the moon in the morning, whereas the moon sets soon after the sun in the evening.
Happily, planning your attempt is made simple enough software such as Stellarium, which is free, or SkySafari 6, which is paid.
We prefer SkySafari 6, and to plan your thinnest crescent hunt using it, follow these instructions:
- Type ‘moon’ into the search box and select it from the dropdown
- Click on the ‘info’ button to see when the next new moon will occur
- Click on the clock next to that date (see image below) which will change the date and time to the moment of the next new moon
- Next, add one day to the time of new (or subtract a day for a morning observation)
- Now move the clock backward or forwards an hour at a time so that the moon is showing above the horizon when the sun is below it
- When you’ve done that, make sure you have the moon highlighted (6a) and centered (6b).
- Press the ‘info’ button (2) once more. Now you’ll see statistics for its illumination (7a) and how high above the horizon it will be at that time (7b)
The numbers in the image below refer to those in the list above. Click the image for a full-screen version.
Examples of Thin Moons
Using the technique above, here are some examples of dates that look good for crescent moon hunting.
This first image shows a particularly good event that happened in January 2021. The moon is less than two days old, is 12° above the horizon just after sunset (from mid-US latitudes) and is only 3.7% illuminated.
We can get younger than that though, but it does get trickier!
This next image shows the moon on November 15, 2020, just minutes after sunset. Our satellite is 4° above the horizon, less than a day old, and less than 1% illuminated.
We have just a few minutes, maybe 15 or 20, to spot a moon like this before it disappears below the horizon.
If you can time your viewing when the moon is closer to Earth (perigee), you’ll have two advantages over when it’s at apogee.
Firstly, the moon will be bigger, making it easier to spot. Secondly, the moon puts distance between itself and the sun much more quickly when closer to earth than when further away, giving us precious extra height and minutes to observe it.
Both the November 2020 and January 2021 new moons shown above happen very close to lunar perigee making them fantastic events to spot.
You are now armed to see the youngest new moon (or the oldest waning moon) you’ve ever achieved.
It’s over to you now, do your planning, get yourself set and go and achieve your own personal record.
What is going to be the youngest new moon you will see?