The longitude coordinate is a vertical line on the globe used to pinpoint a location east or west of the Prime Meridian. When two cities share the exact same longitude, they share a precise geographical and astronomical alignment. This shared coordinate line, known as a meridian, dictates how the cities are situated relative to one another and when they experience the highest point of the sun’s daily path.
Geographical Alignment: The North-South Path
A line of longitude, or a meridian, is a half-circle drawn on the Earth’s surface that extends from the North Pole to the South Pole. Meridians are not parallel lines; they are farthest apart at the equator and converge completely at both poles. When two cities share the same longitude, one is directly due north or due south of the other, tracing the same arc between the poles.
This alignment creates a fixed, directional relationship between the two locations, regardless of the distance separating them. For example, a city in Canada and a city in Brazil could share the same longitude, meaning an imaginary line connecting them would follow a perfect North-South trajectory. Their position along this path is determined by their respective latitudes, which measure distance north or south of the equator.
The Determination of Local Solar Time
The most significant consequence of shared longitude is the synchronization of their Local Solar Time (LST). Local Solar Time is based directly on the sun’s position in the sky at a specific location. Because the Earth completes a 360-degree rotation in 24 hours, it rotates 15 degrees of longitude every hour.
Since both cities lie on the same meridian, they rotate beneath the sun’s path at the same rate, experiencing the same solar angle. This means both locations experience solar noon—the precise instant the sun reaches its highest point in the sky and crosses their shared meridian—simultaneously. Every one degree of longitudinal difference equates to a four-minute difference in solar time, making a shared meridian a zero-difference reference point.
While their solar noon is identical, the exact times of sunrise and sunset will not be precisely the same due to their different latitudes. Latitude affects the length of the day and the angle of the sun’s path. However, the midpoint of the solar day—solar noon—remains perfectly simultaneous for both locations.
Longitude Versus Standard Time Zones
Despite the perfect synchronization of their Local Solar Time, two cities on the same longitude often use different times on their clocks. This is because the Standard Time Zone (STZ) system is a social and political construct, not a purely astronomical one. Standard Time Zones were created to simplify commerce and travel by setting a uniform time across a wide band of territory.
These time zones are theoretical slices, generally 15 degrees of longitude wide, centered on a “standard meridian” which is a multiple of 15 degrees. However, the actual boundary lines are adjusted to follow political borders, state lines, or other geographic features for convenience. This adjustment means a city may be far to the east or west of its time zone’s central meridian, causing its clock time to deviate significantly from its true Local Solar Time.
For example, a city near the eastern edge of its 15-degree time zone slice will experience solar noon almost an hour earlier than a city on the same time zone’s western edge. Therefore, while shared longitude dictates a precise shared solar time, the clocks in the two cities will only match if they fall within the same politically defined Standard Time Zone.