Astronomy Basics
Fundamental concepts of visual astronomy to help with your observations.
Coordinate Systems
Horizontal System (Alt/Az)
The horizontal system describes positions relative to your location:
- Azimuth (Az): Compass direction — 0° = North, 90° = East, 180° = South, 270° = West
- Altitude (Alt): Angle above the horizon — 0° = horizon, 90° = zenith (directly overhead)
StarHop Navigator uses this system because it directly corresponds to your telescope’s orientation.
Equatorial System (RA/Dec)
The equatorial system is fixed to the sky and doesn’t rotate with Earth:
- Right Ascension (RA): Celestial “longitude” (in hours, 0–24h)
- Declination (Dec): Celestial “latitude” (−90° to +90°)
Catalogs use RA/Dec; the app converts these to Alt/Az for your location.
Brightness (Magnitude)
The brightness of celestial objects is measured in magnitudes (mag):
- The smaller the number, the brighter the object
- Sirius: −1.5 mag (very bright)
- Polaris: 2.0 mag
- Naked-eye limit: about 6 mag
- Each step corresponds to a factor of about 2.5 in brightness
Object Types
Stars
Individual points of light. Bright stars serve as calibration stars.
Deep-Sky Objects (DSO)
Objects beyond our solar system:
- Open clusters: Loose groups of young stars (e.g. M45, the Pleiades)
- Globular clusters: Dense, spherical concentrations (e.g. M13 in Hercules)
- Nebulae: Gas or dust clouds (e.g. M42, the Orion Nebula)
- Galaxies: Star systems outside the Milky Way (e.g. M31, the Andromeda Galaxy)
Planets
Wandering stars in our solar system. The app shows Mercury, Venus, Mars, Jupiter, and Saturn.
Catalogs
- Messier (M): 110 of the brightest DSOs, compiled by Charles Messier. Ideal for beginners.
- NGC: New General Catalogue — extensive catalog with thousands of objects.
- Caldwell: 109 objects not included in the Messier catalog. Can be enabled in Settings.
Observing Conditions
Seeing
Atmospheric steadiness — how calm the air is. Good seeing is important for planetary detail and high magnifications.
Transparency
Sky clarity — how clear the atmosphere is. Good transparency is important for faint objects.
Bortle Scale
Light pollution scale from 1 to 9:
- 1–2: Excellent sky (remote areas)
- 3–4: Rural sky (good for DSOs)
- 5–6: Suburban sky (bright objects easily visible)
- 7–9: City sky (only Moon and planets show well)
Mount Types
Azimuthal / Dobsonian
The telescope moves on two axes: left/right (azimuth) and up/down (altitude). StarHop Navigator is optimized for this mount type.
Equatorial
One axis is aligned with the celestial pole. Allows tracking with a single rotation.