South Llano River State Park

Eclipse viewing

Park Alerts...


The Stars at Night are Big and Bright!

Sundet over the Park HQ.

You’ll find lots to do at South Llano River State Park during the day. But the fun doesn’t end when the sun goes down. We have a great view of the night sky, including the Milky Way!

Extend your stargazing past 10 p.m. by adding on the night activity fee when you pay your entry fee!

Check out these free stargazing tools:

How Dark Is It?

The Bortle Dark-Sky Scale measures how well you can see objects in the sky at night. Light pollution and sky glow can interfere with your ability to see “celestial objects.”

The scale ranges from Class 1, the darkest skies available on Earth, through Class 9, inner-city skies. South Llano’s Bortle Scale rating is 3. (Visit Bortle Scale Ratings page to see ratings for all state parks.)

Check out South Llano River Light Pollution Map. The park is at the crosshairs. The map will show the sky glow around the park with colors that match the Bortle scale.

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.

South Llano's clear sky chart

Real-Time Dark Sky Monitoring

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.

Current Reading

On the Putman Mountain Observatory - Hill Country Dark Sky Monitors webpage, you can see a real time sky darkness measurements at South Llano River. 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.

Learn more about these terms on our Astronomy Definitions page.

Current Night’s Readings

On the Putman Mountain Observatory - Hill Country Dark Sky Monitors webpage you can see graphs that 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.

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 South Llano River State Park or see the current moon phase calendar.