Enchanted Rock State Natural Area

Park closures possible on weekends and holidays.

Stargazing

Dark Skies and Bright Stars

Stars over Enchanted Rock.

Enchanted Rock State Natural Area mesmerizes with its geology, legends and natural beauty. Its rural dark sky also makes it one of the best public places for stargazing in Central Texas.

The skies here are still dark enough to view the Milky Way, but they need protection from encroaching city lights. We have taken significant steps to minimize nighttime lighting in the park.

Pick any place around the park to marvel at the vastness of our universe. Visit one of our Rock Star Parties to learn about the constellations and see a shooting star.

Sky Quality Meter

We have installed a dark sky monitor to measure the relative darkness of the sky above the park. The monitoring station posts readings to this page every 15 minutes during the night. The meter is not active during daylight.

Real-Time Sky Darkness Monitoring

Current Reading

Below you will see the real time sky darkness measurements at Enchanted Rock. Remember, the meter does not take readings during the day.

The chart shows the current sky darkness readings in Mag/Sq Arcsec (magnitudes per square arc second) and NELM (naked eye limiting magnitude). The higher the number in Mag/Sq Arcsec and NELM, the darker it is.

A reading greater than 21.5 Mag/Sq Arcsec or 6.0 NELM indicates a very dark sky. Readings greater than 22.5 Mag/Sq Arcsec typically mean that the sky is cloudy.

See What is Magnitude to learn more about these measurements.

SQM Current Readings
SQM Current Readings

 

Current Night’s Readings

The graphs below show fluctuations in sky darkness over time. One chart shows NELM measurements and the other shows Mag/Sq Arcsec. Again, the monitoring station takes readings every 15 minutes throughout the night and posts in real time. 

In these graphs, the blue line represents the sky quality meter reading. The line with white circles shows the altitude of the moon. Look for the current phase of the moon, as a percentage, in the lower right corner of the graph. The blue line will decrease as the moon rises, because as the moon climbs higher in the sky, the sky brightens. Conversely, the blue line will increase as the moon sets:  as the moon gets lower in the sky, the sky darkens.

SQM Nelm Graph
Naked Eye Limiting Magnitude

SQM MPSAS Graph
Magnitudes per Square Arc Second

 

The rising and setting of the moon, the Milky Way passing directly overhead, and man-made light can cause fluctuations in sky darkness. 

To learn more, look up current moon phase information for Enchanted Rock State Natural Area or see the current moon phase calendar.

Bortle Scale

The Bortle Dark-Sky Scale measures the night sky’s naked eye brightness and the stellar limiting magnitude of a particular location. In other words, it quantifies how well you can see celestial objects given the interference from light pollution and sky glow. 

The scale ranges from Class 1, the darkest skies available on Earth, through Class 9, inner-city skies. The map below shows the sky glow around the park with colors that match the Bortle scale.

Enchanted Rock’s Bortle Scale rating is 3. Visit Bortle Scale Ratings page to see ratings for all state parks.

ERSNA Light Pollution Map
ERSNA Light Pollution Map. Enchanted Rock is designated at the ERSNA crosshairs.

 

Clear Sky Chart

A Clear Sky Chart is an astronomer’s forecast about sky conditions, including, darkness, cloudiness, transparency and the seeing quality. More information about the Clear Sky Chart can be found here.

Related Links:

What is Magnitude?

Both NELM and Mag/Sq Arcsec use “magnitude,” which comes from the term “apparent magnitude,” used by astronomers to describe the brightness of an object in the night sky. Apparent magnitude originally used a scale from 1 to 6, with 1 representing the magnitude of a particular reference star and 6 representing the faintest object that can be seen by the naked eye.

Each level represents a change in brightness of 2.5 times. So an object of magnitude 3 is 2.5 times brighter than an object of magnitude 4. The important point to remember is that brighter objects have smaller magnitudes and fainter objects have larger magnitudes. Very bright objects have negative numbers. For example, the brightest star in the sky is Sirius with an apparent magnitude of -1.6. Sirius is near the constellation Orion in the winter sky.

Some examples of objects with varying magnitudes are listed below.

Apparent MagnitudeCelestial Object
-26.7 Sun
-12.6 Full Moon
-4.4 Venus (at brightest)
-3.0 Mars (at brightest)
-1.6 Sirius (brightest star)
+3.0 Naked eye limit in an urban neighborhood
+5.5 Uranus (at brightest)
+6 Naked eye limit
+9.5 Faintest objects visible with binoculars

NELM

Naked Eye Limiting Magnitude or NELM represents the faintest magnitude that your naked eye can see in the conditions being measured. A NELM of 6.0 means you will be able to see objects as faint as magnitude 6.0; this is about as faint as the human eye can see. On the other hand, a NELM of 2.0 means the sky is relatively bright. This could be due to the moon or your location, say in an urban area with lots of light pollution. 

Some of the darkest locations on Earth have a NELM of around 7.0 to 7.5 - very dark, indeed.

Mag/Sq Arcsec

Astronomers also use Mag/Sq Arcsec to measure relative sky darkness. Mag/Sq Arcsec is the apparent magnitude for a certain square area of the sky measured in arc seconds. 

A square arc second is a square area of the sky that has one arc second on each side. An arc second is a very small measurement. For example, Jupiter looks very small in the sky, but through binoculars it does have a disk-like shape and a small diameter. Jupiter’s angular diameter ranges from 30 to 50 arc seconds. 

Therefore, Mag/Sq Arcsec means the brightness (expressed in apparent magnitude) of a one square arc second area of the sky.


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