To solve this question, I think of my dish as fixed on the sky at az 168° / el 75°. Over 24 hours, the Earth’s rotation sweeps different parts of the Milky Way through that beam, and my hydrogen line trace reflects how much structure lies along that line of sight.
The key point is this:
- Two strong peaks (“two arms”) → I am looking through the inner Galaxy, where multiple spiral arms overlap along the line of sight.
- One peak (“one arm”) → I am looking outward toward the outer Galaxy, where there’s less structure in that direction.
🌌 Inner Galaxy (two arms visible)
When my trace shows two distinct hydrogen peaks, my beam is intersecting:
- The direction toward the Galactic Centre (roughly Sagittarius/Scutum region)
- Multiple spiral arms stacked along the same sightline
- Gas moving at different velocities → gives separate spectral peaks
This corresponds to Galactic longitudes roughly:
- l ≈ 0° → ±60°
So, at those times in my 24-hour transit, my telescope is effectively looking into the crowded, inner Milky Way.
🌌 Outer Galaxy (single arm visible)
When my trace shows a single hydrogen peak, I am looking:
- Away from the Galactic Centre
- Toward the Galactic anticentre (Auriga/Perseus direction)
- Through fewer spiral arms (often just the local + one outer arm)
This corresponds roughly to:
- l ≈ 120° → 240°
So the signal simplifies to one dominant velocity component.
🧭 How the above maps onto my fixed pointing
At az 168° / el 75° (UK latitude):
- I am scanning a narrow declination band near +20° to +30°
- Over 24 hours, that cuts across:
- Cygnus region → rich structure (often 2 components)
- Cassiopeia / Perseus arm → usually 1 dominant component
- Occasionally toward inner-arm tangents → 2 peaks again
🧠 Practical rule of thumb
- Two peaks = inner Galaxy (toward Galactic Centre side of the sky)
- One peak = outer Galaxy (toward anticentre side)
Turning the fixed pointing of LRO-H2 into something I can directly use alongside my data.
🧭 Step 1: What my telescope is actually scanning
At Lichfield (~52.7° N) with:
- Azimuth = 168° (almost due south)
- Elevation = 75° (very high)
my beam sits at a declination ≈ +28° (very close approximation).
As the Earth rotates, I sweep through all right ascensions at Dec ≈ +28° once every 24 hours.
🌌 Step 2: Convert that path into Galactic longitude
That declination track crosses the Milky Way at an angle, intersecting different Galactic longitudes over time.
Here’s a practical “observer’s timeline” for my setup:
⏱️ 24-hour interpretation guide (approximate)
| Local Sidereal Time (LST) | Sky Region | Galactic Direction | What I see |
| ~18h–21h | Cygnus | l ≈ 70°–90° | Two components (multiple arms) |
| ~21h–00h | Cepheus / Cassiopeia | l ≈ 90°–120° | Transition → often one dominant |
| ~00h–04h | Perseus / Auriga | l ≈ 130°–170° | One peak (outer Galaxy) |
| ~04h–08h | Taurus / Gemini | l ≈ 170°–210° | Single weak/broad peak |
| ~08h–12h | Monoceros | l ≈ 210°–240° | Outer Galaxy, still mostly one |
| ~12h–16h | Canis Minor → Hydra | l ≈ 240°–300° | Signal weakens (off plane) |
| ~16h–18h | Back toward Cygnus | l ≈ 50°–70° | Two peaks return |
🔑 What controls the number of peaks
When I see TWO peaks
I am looking along a direction where my line-of-sight cuts across multiple spiral arms at different velocities, typically:
- Cygnus region (local + Perseus arm)
- Tangential directions to arms
When I see ONE peak
I am looking:
- Outward from the Galaxy
- Through fewer structures
- Mostly the local arm + one outer arm blended
🧠 The important insight
My scan does NOT pass directly through the Galactic Centre (that’s at Dec ≈ –29°, below my horizon peak), but:
- I still sample inner-arm tangents → gives two spectral components
- And outer Galaxy directions → gives one component
📈 How to label my data
If my data is timestamped, I can:
- Convert time → Local Sidereal Time
- Use this quick rule:
- LST ~18h–21h → TWO peaks (Cygnus arm region)
- LST ~00h–08h → ONE peak (outer Galaxy)
- LST ~16h–18h → TWO peaks again