what is umbral depth

What is Umbral Depth? Everything You Need to Know!

All the eclipse fans are always enticed about the next eclipse. Yet, many people do not acknowledge the umbral aspects. 

For proper visuals of an eclipse, one needs to understand the umbral depth of the eclipse. 

So, what is umbral depth

The shadow of the moon that’s cast on earth while the eclipse happens is the umbra. And the depth of that shadow derives from the word ‘umbral depth’. This determines the totality and duration of an eclipse from the perspective of an observer. There are many formulas used to calculate the umbral depth. 

This portion is only a fraction of the information you need to know. You’ve got to read the full article to get a better understanding. 

Start reading here. 

What Is an Umbra?

Before diving in to know what umbral depth is, we need to cover the basics of it. Knowing what an umbra is important to grasp and comprehend the umbral depth.

So, what is an umbra?

In simple words, the umbra is a shadow of an object. However, there are many levels or shades of shadow. The umbra is the deepest part of a shadow. The term ‘umbra’ is mainly used to describe the sunspots. This term is only viable for non-transparent objects though!

The antumbra, umbra and penumbra are the 3 parts of a shadow. The different types of eclipses can be considered among the famous umbra examples. There are two shadows cast when an eclipse happens. One of them is an umbra.

umbra and penumbra
Source: timeanddate.com

Observers under the penumbra experience partial eclipses. Also, the observers under the antumbra experience annual eclipses.

Due to the umbra, we can see blood moons. It happens when the earth cast a shadow on the moon. So the moon doesn’t glow and emits a low-level light; that looks reddish-orange.

Now we know about the umbra. Let’s learn about the umbral depth! I’ve included a proper explanation for it in the next part. Read along to find out.

What Is Umbral Depth?

Many people understand the concept of umbra but fail to understand the umbral depth. If you’re one of them, I got you covered.

Umbral depth
Source: nasa.gov

Umbral depth is basically the difference between the totality path and the center line. Note that, along with the distance, it also measures the duration. When there’s an umbra cast on the earth’s surface, you can measure the umbral depth. 

So if you consider the lunar eclipse, the moon is within the earth’s umbra. The center of that umbra is the totality path. Being under the totality path will result in the maximum umbral depth and eclipse duration.

In the totality path, the images of the eclipse are the most vivid. If you’re planning to take pictures, you can use either CMOS or DSLR cameras. 

For instance, the penumbra sunspot’s umbral depth is less. It’s because this area is a bit far from the totality path. The more you stray away from the path, the more the umbral depth will reduce. 

To know the umbral depth, you have to know to calculate and take measurements of it.  Don’t know how to do it? No worries! 

Read the next section to know how to calculate the umbral depth of a location. It’s an in-depth guide to it, so do the calculation with care!

How to Know Umbral Depth of Your Location?

Since the eclipses switch places for every instance, the umbral depth needs to be calculated accordingly, There’s a formula to calculate the umbral depth. Buckle up because we’re going to do some serious math!

So, what is the umbral depth formula?

The umbral depth formula is the formula you can use to determine a location’s umbral depth. A couple of pieces of information are required for this formula to work. For example, the perpendicular distance and the umbral shadow radius. The formula consists of all these terms. 

The formula looks like this,

U = 1-|x/R|

Here, U is the umbral depth. Also,  x is the perpendicular distance. This distance is calculated from the shadow axis. Lastly, the R is the radius of the umbral shadow.

You can calculate the umbral depth in miles or kilometers. It is really up to you! Here the more umbral depth you have, the more center of the totality path you’re in. 

Umbral depth directly affects the duration of the totality path. You can calculate the duration of the totality path by this formula-

D = d/(1-(1-u)2)1/2

Here, D is the total duration of the totality. It’s calculated on the central line. Also, d is the duration of the totality at your location. Lastly, u is the umbral depth here. The duration is in second here. 

You can only proceed to calculate with this formula when you’re done calculating the umbral depth. 

As you know, lunar eclipses last for 2 hours. So in that regard, the output of this equation will be 7200 (seconds).

To maximize totality duration, it’s not mandatory to stay at the centerline. You be a bit off and still get 85-90% umbral depth!

Umbral depth is mainly measured when observing an eclipse. By measuring the umbral depth of an umbra, you can know what region you’re under. 

For instance, the accuracy of the eclipse will differ when comparing 50% and 90% umbral depth. Obviously, if you’re in 90% umbral depth, you’ll get a better eclipse viewing experience.

For viewing the eclipse, you can use binoculars if you want. A midrange size of binoculars may do the job. For example, you can choose between 10×50 and 15×70 binoculars

To be safe, get some eclipse glasses before doing this. View through the telescope or binocular of your choice; while wearing the eclipse glasses. It’s applicable for partial, non-partial, and full eclipses.

Want some really good eclipse glasses for your Eclipse viewing? Check out these amazing recommendations that we have: 

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By using these, you can enjoy many more sightings including the annual, total, and partial eclipses!

Frequently Asked Questions (FAQs)

What Occurs More Often: A Partial Solar Eclipse or a Total Solar Eclipse?

One can observe the partial solar eclipse multiple times a year. Partial eclipses are far more frequent when compared to full solar eclipses. A partial solar eclipse occurs before the full one. It is seen roughly once every two months. Also, the umbral depth of partial eclipses is less than full solar eclipses. 

What Eclipse Happens Every 50 Years?

The total solar eclipse happens once every 50 years. All countries can see the eclipse, it is not limited to regions. The next total eclipse will occur someday between 2067. Other counts of partial and full eclipses will add up to 21 before 2067. So if you’re waiting for a total eclipse, you’re in luck!

How Often Does the Blood Moon Appear?

Blood moons come around twice a year. Very rarely, the blood moon happens 3 times a year. The blood moon happens when the umbral of the earth falls on the moon. In this state, the moon is totally excluded from the sunlight. Hence, it turns reddish-orange instead of the regular off-white color. 


This is the end of this article. Hopefully, now you know what is umbral depth. The information provided in this article should be enough to know it!

There are many umbral depth calculators out there. You can skip the calculations this way.

Have fun determining the umbral depth!

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