What is the radiant point of a meteor shower?

The radiant is the point in the sky from which meteors appear to originate. It is defined by a right ascension and declination. The higher the radiant is above the horizon, the more meteors you will see per hour because more of the sky is exposed to incoming particles.

What is hour angle and how do I find it?

Hour angle (HA) measures how far the radiant has traveled past your local meridian, in hours. Negative values mean the radiant is still rising (east of meridian); positive values mean it is descending (west of meridian). HA = 0 means the radiant is transiting — at its highest point. You can calculate HA as Local Sidereal Time minus the radiant's Right Ascension.

Why does the calculator use declination but not right ascension?

Altitude depends on your latitude, the declination of the object, and the hour angle. Right ascension only determines when the object crosses the meridian; it is already embedded in the hour angle you provide. So declination and hour angle together are sufficient to compute altitude.

What altitude gives the best viewing?

A radiant altitude above 30° is considered good; above 50° is excellent. Below 20° atmospheric extinction and a long path through the atmosphere significantly reduce observed meteor rates. Below 0° the shower has not yet risen above your horizon.

Can southern hemisphere observers use this?

Yes. Enter your latitude as a negative number (e.g., -34 for Sydney). The formula works correctly for all latitudes. Some northern showers like the Perseids have a negative southern-hemisphere altitude even at transit, meaning they never rise for far-southern observers.

What is Zenith Hourly Rate (ZHR) and is it shown here?

ZHR is the theoretical maximum meteor count per hour when the radiant is at the zenith under perfect conditions. This tool calculates radiant altitude, not ZHR. However the viewing quality rating shown gives you a practical sense of how activity scales with altitude.

Meteor Shower Radiant Calculator

Name: Meteor Shower Radiant Calculator
Availability: InStock
Author: Nham Vu

Enter your latitude, the radiant declination, and hour angle to find how high the meteor shower radiant sits above your horizon.

Observer & Radiant Inputs

Fill in your location and the radiant coordinates, then press Calculate.

Quick-fill a meteor shower

Observer Latitude (degrees, negative = south)

Radiant Declination (degrees, negative = south of equator)

Hour Angle (hours; negative = rising, 0 = transit, positive = setting)

HA = Local Sidereal Time − Right Ascension

Enter your inputs and press Calculate to see the radiant altitude.

Summary

Enter your latitude, the radiant declination, and hour angle to find how high the meteor shower radiant sits above your horizon.

How it works

Enter your observer latitude and the radiant declination.
Enter the hour angle (hours east or west of your meridian).
The tool applies the altitude formula: sin(alt) = sin(lat)·sin(dec) + cos(lat)·cos(dec)·cos(HA).
Azimuth is derived from the same spherical triangle.
A viewing quality rating (Excellent / Good / Fair / Poor / Below Horizon) is assigned based on the resulting altitude.

Use cases

Determine the best time of night to watch a meteor shower from your location.
Check whether the Perseids, Geminids, or Leonids are visible from your latitude.
Plan an astrophotography session around peak radiant altitude.
Teach students how radiant altitude depends on latitude and time of night.
Compare viewing conditions for observers at different latitudes.
Verify published optimal viewing windows against your local geometry.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu