Often, despite its imposing presence crossing the celestial vault, the Milky Way escapes our gaze when we contemplate the night sky. What is the reason?
The Milky Way does not appear the same depending on where you are on Earth. This spectacular variation results from the combination of the terrestrial latitude and the orientation of the galactic plane. Contrary to popular belief, it is not just light pollution that affects its visibility, but fundamental astronomical parameters.
The galactic center (located in Sagittarius and Scorpius) has a declination of about −29°. This means it is intrinsically in the southern celestial hemisphere.
N.B.:
The declination is the angle, denoted δ, measured on the celestial sphere between an object and the celestial equator. A positive declination means the object is in the northern celestial hemisphere, and a negative declination means it is in the southern celestial hemisphere. The galactic center has a declination of about δ = −29°, which explains why it is always better visible in the southern hemisphere, where it can reach an altitude close to the zenith.
| Latitude | Hemisphere | Milky Way Visibility | Appearance at 00:00 | Appearance at 04:00 | Maximum Altitude of the Galactic Center | Comment |
|---|---|---|---|---|---|---|
| 0° | Equator | Excellent | Vertical ~90° | Inclined ~60° | ~61° | The galactic plane passes high in the sky, almost at the zenith |
| +15° | North | Very good | Very vertical ~80° | Inclined ~50° | ~46° | The galactic center is clear but not dominant |
| -15° | South | Excellent | Very vertical ~85° | Inclined ~55° | ~76° | The galactic core approaches the zenith |
| +30° | North | Very good | Vertical to inclined ~70° | Inclined to lying ~40° | ~31° | Center still low, moderate visibility |
| -30° | South | Exceptional | Very vertical ~90° | Inclined ~70° | ~89° | The central bulge almost at the zenith |
| +45° | North | Good | Inclined ~60° | Lying ~30° | ~16° | Center very low, reduced visibility |
| -45° | South | Excellent | Vertical ~80° | Inclined ~50° | ~61° | Center high and contrasted |
| +60° | North | Average | Inclined ~45° | Lying ~20° | ~1° | Galactic center almost at the horizon |
| -60° | South | Very good | Vertical ~75° | Inclined ~55° | ~59° | Excellent exposure to the central bulge |
| +75° | North | Poor | Lying ~10° | Lying ~5° | ~0° | The center remains below or on the horizon |
| -75° | South | Good | Inclined ~40° | Lying ~15° | ~44° | Still good access to the heart of the galaxy |
| +90° | North Pole | Very poor or impossible | Lying ~0° | Lying ~0° | ~−29° | Galactic center below the horizon, invisible |
| -90° | South Pole | Poor | Lying ~10° | Lying ~5° | ~13° | The galactic center is low but partially observable |
The observer's latitude is decisive; it defines which portion of the sky is visible at any given time.
From the equator (latitude 0°), an observer can, over the year, see all parts of the celestial vault, as the Earth's rotation and its revolution around the Sun allow each region of the night sky to pass above the horizon. At the equator, the galactic plane can appear perpendicular to the horizon, offering a complete view of the disk.
At 45° north latitude, an observer can, over the year, see most of the celestial vault, but some regions near the southern celestial pole always remain below the horizon and are never observable, while the Earth's rotation and its revolution around the Sun allow the rest of the night sky to pass above the horizon. At mid-latitudes (such as in Europe or North America), the Milky Way appears inclined, sometimes low on the horizon.
At the North Pole (90° latitude), most of the sky rotates around the zenith, and some regions remain perpetually above or below the horizon, preventing the observation of the entire celestial vault over the year. In these extreme latitudes, the galactic center often remains invisible all year round.
The galactic plane of the Milky Way is inclined by about \(60^\circ\) relative to the ecliptic plane (the plane of Earth's orbit). This orientation is fixed in space and determines the relative position of the galactic plane for any observer on Earth.
The position of the galactic center is particularly important. Located in the constellation Sagittarius with a declination of about \(-29^\circ\), it represents the most spectacular region of the Milky Way, but its visibility strongly depends on the latitude:
This north/south asymmetry is therefore not due to a change in the galactic plane, but to its actual position in the sky relative to the celestial equator and the observer's latitude.
The annual revolution of the Earth means that the night sky visible at midnight changes every month. Each season, the Milky Way appears with a different inclination relative to the horizon. The illusion of rotation or angle change in the sky is a combination of Earth's rotation (day/night) and Earth's revolution (seasons).
The Earth completes one revolution around the Sun in ~365.25 days. At local midnight, the Earth-Sun direction (the line connecting the Earth to the Sun at that moment) is opposite the observer's meridian. The angle between this direction and the local meridian changes by about 1° each night due to the Earth's annual revolution in the ecliptic plane. This daily variation means that different portions of the galactic plane pass the meridian throughout the year, explaining the seasonal evolution of the Milky Way's visibility.
After about 30 days, this shift reaches ~30°, which roughly corresponds to the apparent width of a major constellation. This means that each month, the night sky visible at midnight presents a significantly different portion, enough to observe new parts of the Milky Way.
| Latitude | Season | Galactic Center Visible | Approximate Inclination | Comment |
|---|---|---|---|---|
| 0° (Equator) | Spring | Sagittarius low in the south | ~45° | Galactic plane inclined, center quite high |
| 0° (Equator) | Summer | Sagittarius near the zenith | ~90° | Galactic plane almost vertical |
| 0° (Equator) | Autumn | Sagittarius descending | ~45° | Galactic plane inclined towards the southern horizon |
| 0° (Equator) | Winter | Cygnus at the northern zenith | ~90° | Galactic plane almost vertical on the north side |
| 45° N | Spring | Sagittarius low on the southern horizon | ~20°–30° | Galactic center low, reduced visibility |
| 45° N | Summer | Sagittarius higher | ~60–70° | Galactic plane strongly inclined |
| 45° N | Autumn | Sagittarius descending | ~30–40° | Galactic center low |
| 45° N | Winter | Cygnus dominant in the north | ~70–80° | Galactic plane almost vertical on the north side |
| 45° S | Spring | Sagittarius high in the south | ~70–80° | Galactic center near the zenith |
| 45° S | Summer | Cygnus low in the north | ~30–40° | Galactic plane inclined towards the northern horizon |
| 45° S | Autumn | Sagittarius descending | ~60–70° | Galactic center well exposed |
| 45° S | Winter | Cygnus low in the north | ~20–30° | Galactic plane very inclined, center low |
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