Precise moon ephemerides including phase and illumination (astronomy-engine Illumination and MoonPhase)
Real-time planetary ephemerides
Visibility Calculator
Calculate when celestial objects are visible from your location throughout the year
Getting Started
Quick Start
Search for an object → Set location → Click Calculate → Expand Altitude Plot for detailed single-night view
What You'll Get
Yearly Visibility Plot: Theoretical imaging hours per night across all 12 months
Altitude Plot: Detailed single-night view with twilight zones and moon data
2D Sky Path: Polar plot showing object trajectory across the sky
3D Sky Sphere: Interactive hemisphere visualization
Time Animation: Watch object movement throughout the night
Target Object Selection
Catalog Search
Search more than 17,100 astronomical objects across the major catalogs:
Messier (M): M1-M110 (e.g., M31 Andromeda Galaxy)
NGC: New General Catalogue (e.g., NGC 6822)
IC: Index Catalogue (e.g., IC 1805 Heart Nebula)
Sharpless (Sh2): Emission nebulae (e.g., Sh2-308)
Caldwell (C): C1-C109 (e.g., C14 Double Cluster)
Others: Barnard, Abell, VdB, LBN, LDN, PGC
Examples:
M31, NGC 6822, IC 1805, Sh2-308, C14
Manual Coordinates
Enable "Use Manual Coordinates" to enter custom celestial coordinates:
Right Ascension (RA)
Decimal hours:10.6847 (0-24 hours)
HMS format:10h 41m 4.5s
Declination (Dec)
Decimal degrees:41.2686 or -41.2686
DMS format:+41° 16' 8.4"
Planet & Moon Tracking
Enable "Use Planet / Moon" to track solar system objects with precise ephemerides:
The Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto
Real-time ephemerides updated at each time step
Accounts for orbital motion throughout the year
Note
Planet mode overrides catalog search and manual coordinates. When the Moon is the target, the Moon filters are disabled (they would exclude the Moon itself).
Observation Locations
Latitude
Range: -90° to +90°
Positive: Northern Hemisphere
Negative: Southern Hemisphere
New York: 40.7
Tokyo: 35.7
Sydney: -33.9
London: 51.5
Longitude
Range: -180° to +180°
Positive: Eastern Hemisphere
Negative: Western Hemisphere
Quick Location
Click "Use My Location" to automatically detect your coordinates. Your location is only used locally in your browser.
Multiple Locations
Compare visibility from different locations using comma-separated values:
Minimum altitude the target must reach above the horizon:
Altitude
Recommendation
Use Case
0°-15°
Not recommended
Heavy extinction, distortion
20°
Minimum acceptable
Moderate extinction
30°
Good quality
Less extinction, faint objects OK
45°+
Excellent
Minimal extinction, best quality
Sun Altitude Threshold
Maximum sun altitude for acceptable dark-sky conditions:
Sun Altitude
Twilight Type
Imaging Conditions
0° to -6°
Civil Twilight
Too bright for most imaging
-6° to -12°
Nautical Twilight
Bright objects only
-12° to -18°
Astronomical Twilight
Good for deep-sky
Below -18°
True Night
Optimal for faint objects
Common Values:
-12° = Nautical twilight (relaxed)
-15° = Mid-astronomical (balanced)
-18° = True night (strictest)
Moon Filters
Optional filters to find optimal dark-sky conditions. Both can be enabled independently.
Moon Altitude Filter
Excludes times when the Moon is above a specified altitude:
0°: Most restrictive - Moon below horizon
15°: Moderate - Low moon acceptable
30°+: Relaxed - Moon can be moderately high
Moon Separation Filter
Excludes times when the Moon is closer than specified angular distance:
30°: Minimum recommended
45°: Good separation
60°+: Optimal - minimal interference
Ignore a Faint Moon
Optional: on dates when the Moon is less than the chosen percentage illuminated (default 25%), the Moon altitude and separation filters are skipped for that date, so thin-crescent nights aren't excluded. It has no effect unless one of those filters is enabled.
15%: Ignore only very thin crescents
25%: Roughly 3–4 days either side of new Moon
40%: About 5–6 days either side of new Moon (narrowband)
The illumination for a date is taken at the site's local noon (the middle of that date's 24-hour window) and is decided separately for each location.
Tip
Use 0° moon altitude for faint nebulae. Use 15°-30° for bright objects or narrowband imaging.
Understanding Results
Quality Imaging Hours Plot
Shows theoretical usable hours per night across the entire year:
X-axis: Months (January - December)
Y-axis: Hours per night meeting all criteria
Peak months: Best months for imaging this target
Moon strip: Thin band under the plot shaded by Moon illumination (dark = new Moon, pale = full Moon); the hover shows the phase, e.g. "Moon: Waxing Gibbous (78%)"
Altitude Plot
Detailed view of a single night with color-coded twilight:
White = Daytime
Light blue = Civil twilight
Medium blue = Nautical twilight
Dark blue = Astronomical twilight
Black = True night
The title also shows the Moon phase at local midnight for the selected night.
Sky Path Visualizations
2D Polar View
Horizon = outer ring, Zenith = center
N/E/S/W compass markers
Time slider for animation
3D Sphere View
Interactive hemisphere
Click and drag to rotate
Scroll to zoom
Tips & Common Scenarios
Faint Nebula Imaging
Target Altitude: 30° (minimize extinction)
Sun Altitude: -18° (true night only)
Moon Altitude: 0° (no moon)
Moon Separation: Not needed
Positive: Northern Hemisphere (e.g., 42.0 for New York)
Negative: Southern Hemisphere (e.g., -33.9 for Sydney)
Observer Longitude
Range: -180° to +180°
Positive: Eastern Hemisphere
Negative: Western Hemisphere
Observation Date
Date for the observation session. Calculations cover the entire night from sunset to sunrise.
GPS Location
Click "Use My Location" for automatic coordinate detection.
Understanding Sky Direction Parameters
The Object Finder uses a celestial coordinate system to locate objects in the sky. Three parameters work together to define your search area:
1. Azimuth (Compass Direction)
Range: 0° to 360°
Azimuth is the compass direction measured clockwise from North. Think of it as looking at the sky from above.
Azimuth: View From Above (Looking Down at Observer)
Looking down from above, azimuth sweeps clockwise from North (0°) through East (90°), South (180°), and West (270°).
Interactive Compass
Use the visual compass rose in the app to click and select your azimuth direction instead of typing degrees.
3D Sky Preview
The Object Finder features a real-time 3D sky preview that shows the exact search area BEFORE you click "Find Objects". This interactive hemisphere visualization updates instantly as you adjust azimuth, altitude range, and search radius parameters. Drag to rotate the view, scroll to zoom. The camera position is preserved as you change parameters, so you can explore from your preferred angle. The bounded search patch appears as a colored area on the hemisphere, showing you precisely which part of the sky will be searched.
2. Altitude Range (Elevation Angle)
Min Altitude: 0° to 85°Max Altitude: 5° to 90°
The Object Finder uses an altitude range to define the vertical bounds of your search area. Instead of searching at a single elevation, you specify minimum and maximum altitudes to create a bounded patch in the sky.
Min Altitude
The lower boundary of your search area. Objects below this altitude will not be included in results.
Max Altitude
The upper boundary of your search area. Objects above this altitude will not be included in results.
5° Minimum Gap
The system enforces a minimum 5° gap between min and max altitude to ensure a usable search area. When you adjust one slider, the other may automatically adjust to maintain this gap.
Altitude: Side View (Elevation from Horizon)
From the observer's position: 0° = horizon (looking straight ahead), 45° = halfway up, 90° = zenith (straight overhead).
Altitude Range Examples:
30°-35° = Narrow low-altitude band (5° span, more atmospheric extinction)
20°-40° = Moderate elevation band (20° span)
45°-60° = Mid-sky band (15° span, good imaging conditions)
60°-75° = High altitude band (15° span, excellent conditions)
85°-90° = Near-zenith band (5° span at minimum gap, optimal conditions)
3. Search Area Radius
Range: 1° to 90°
The search radius defines how far left and right from your target azimuth to search. Combined with the min/max altitude range, this creates a bounded rectangular patch on the sky hemisphere.
How the Bounded Search Area Works
Your search area is defined by four boundaries:
Left boundary: Target azimuth - search radius
Right boundary: Target azimuth + search radius
Bottom boundary: Minimum altitude
Top boundary: Maximum altitude
Visualize Your Search Area
The 3D Sky Preview shows this bounded rectangular patch in real-time. Watch it update as you adjust the search radius and altitude range to see exactly which part of the sky will be searched.
Search Area: Interactive 3D Sky Hemisphere
Interactive 3D visualization - Drag to rotate, scroll to zoom. The hemisphere shows the sky dome from the observer's perspective. Different colored patches show search areas with varying radii: 15° (red/narrow), 30° (yellow/standard), and 60° (green/wide). Each patch is bounded by min/max altitude (30°-40°) and extends ±radius from the target azimuth (South/180°). Toggle patches on/off using the legend.
Search Radius Guide:
10-15° = Narrow azimuth range for specific target framing (±10-15° from center)
20-30° = Typical wide-field imaging area (±20-30° from center)
40-50° = Large region survey (±40-50° from center)
60°+ = Very broad exploration of entire sky area (±60°+ from center)
Putting It All Together
These three parameters work together to define a bounded rectangular search area on the sky hemisphere:
Complete Example: South at 25°-50° altitude range, 30° search radius
Interactive 3D example - Drag to rotate, scroll to zoom. This shows all parameters working together: Azimuth (South/180°) sets the compass direction, Min/Max Altitude (25°-50°) defines the vertical bounds (25° vertical span), and Search Radius (30°) defines how far left and right to search (±30° in azimuth). The yellow rectangular area on the hemisphere shows the bounded search patch - note how it's clearly bounded by min altitude at the bottom (25°), max altitude at the top (50°), and extends from 150° to 210° in azimuth (±30° from South). The blue diamond marks the center of the patch.
Practical Examples
Example 1: Imaging the Milky Way Core (Northern Hemisphere Summer)
Azimuth: 180° (South)
Min Altitude: 20°, Max Altitude: 40° (low to mid sky band)
Search Radius: 45° (wide field to capture the region) → Finds objects in the southern Milky Way region within a 20° vertical band
Example 2: Overhead Targets at Zenith
Azimuth: 0° (any direction - doesn't matter at zenith)
Min Altitude: 85°, Max Altitude: 90° (near-zenith band at minimum gap)
Search Radius: 30° (moderate area around zenith) → Finds objects passing directly overhead (best conditions, minimal extinction)
Example 3: Eastern Sky in Early Evening
Azimuth: 90° (East)
Min Altitude: 30°, Max Altitude: 60° (low to high elevation band)
Search Radius: 30° (standard wide field) → Finds objects rising in the east across a 30° vertical range
Example 4: Exploring the Northern Sky
Azimuth: 0° (North)
Min Altitude: 50°, Max Altitude: 75° (high in the sky)
Search Radius: 60° (very large survey) → Broad search of northern circumpolar region across 25° vertical span
Pro Tips
Use the 3D Sky Preview! Watch the bounded search area update in real-time as you adjust parameters
Start with azimuth to pick your compass direction (use the interactive compass)
Set min/max altitude to define your vertical search boundaries (minimum 5° gap enforced)
Adjust search radius to control azimuth spread (±radius from target direction)