Moon Conditions & Aurora Viewing
The moon is the aurora watcher's frenemy. A new moon gives you the darkest possible skies — perfect for picking up faint aurora. A full moon washes out the sky and makes subtle displays harder to see. But here's the good news: the moon never completely ruins your chances.
How Lumina scores the moon
Lumina looks at three things when assessing the moon's impact on your viewing:
- Illumination — what fraction of the moon's disk is lit (0 = new moon, 1 = full moon).
- Altitude — how high the moon is above the horizon. A full moon at 60° altitude is much brighter than a full moon at 5°.
- Timing — is the moon up during the dark window? Moonrise and moonset times determine whether it's even a factor.
The brightness model
Lumina models moonlight intensity as:
\[ \text{moon brightness} \approx \text{illumination}^{1.3} \times (0.5 + 0.5 \times \sin^2(\text{altitude})) \]
The illumination exponent (1.3) means a half-moon (50% illuminated) produces less than half the brightness of a full moon — about 38%, to be precise. That's because the moon's surface isn't uniformly reflective; the edges are darker than the centre.
The altitude term captures the fact that moonlight, like sunlight, is brightest when the moon is overhead and dimmest when it's near the horizon, where it passes through more atmosphere.
Moon sentiment scoring
Lumina converts moon conditions into a 0–100 sentiment score where 100 = "perfect" (new moon, not up) and 0 = "worst case" (full moon overhead):
| Condition | Illumination | Sentiment | Badge |
|---|---|---|---|
| New moon | 0–5% | 95–100 | "Moon OK" (green) |
| Crescent | 5–15% | 80–95 | "Moon OK" (dim) |
| Quarter | 15–40% | 45–80 | "Moon X%" |
| Gibbous | 40–90% | 10–45 | "Moon X%" |
| Full | 90–100% | 0–10 | "Moon X%" (orange/red) |
The 0.3 floor — why the moon doesn't kill aurora
This is important: Lumina's moon factor bottoms out at 0.3 , not zero. Even with a full moon directly overhead, the sky brightness model only suppresses aurora visibility by about 70%.
Why? Because bright aurora — especially during substorms — is surprisingly resilient against moonlight. The greens and reds of aurora are emission lines, not broadband light. They're concentrated in narrow wavelengths that can still punch through moonlit skies. A G2+ storm during a full moon is absolutely worth going out for. A quiet arc during a full moon, not so much.
Timing around the moon
The moon's schedule matters just as much as its brightness. Lumina computes your complete darkness window — the periods when the sky is dark enough for viewing — by sampling sky brightness every 5 minutes through the night and finding continuous stretches above the darkness threshold.
If the moon sets partway through the night, you'll get a moonset-qualified window: "Good conditions after moonset at 1:30 AM." If the moon rises during the night, the recommendation will flag the pre-moonrise window as your best bet.
Planning around the moon
If you're serious about aurora watching, plan your outings around the lunar cycle:
- Best: new moon ± 3 days. The moon is either not up at all or a thin crescent that sets early.
- Good: first or last quarter, especially if the moon sets before midnight or rises after 3 AM.
- Still worth it: gibbous moon, if the aurora probability is high and the moon is low in the sky or sets during the dark window.
- Toughest: full moon that's up all night. But even then, a G3+ storm will be visible — you'll just lose the faint detail.