A Closer Look at Rise, Set and Twilight


Astronomy is the study of the universe. Our location in the universe is in a spiral galaxy called the Milky Way. Our solar system, housing just one star, the sun, eight planets and a few smaller entities, which are not planets, but more like moons, orbit it in elliptical, tilted orbits. Our home planet, Earth, lies 93 million miles away from the sun and has one satellite, the moon. The sun lights half of Earth at one time, giving us day. The moon lights the other half of Earth, giving us night. Humans have always looked to the sun and the moon to know what day and time it is, even what season it is.

The Horizon

The sun and the moon are viewable with the naked eye, although no one should look directly at the sun as it is too bright. Humans live on the surface of the earth and by looking at any angle, can see only half of it, making it seem flat. The flat “line” that we see at the bottom of our viewing area is called the nadir. The top of it is called the zenith, and everything in between is called the horizon.

Think of the earth from its North to South poles on its top and bottom as an imaginary vertical line of which is cut in half by an imaginary horizontal line called the equator. The vertical lines are called longitudinal lines and the horizontal lines are called latitudes. We divide the earth into time zones based on those imaginary lines. Since the earth orbits the sun, and rotates in that orbit on a tilted axis, sunrise and sunset advance to the observer on earth by a gradual lightening and darkening of the horizon dependent on where your time zone and location lie. Each day is 23 hours and 56 minutes. Depending on the season, the time of daylight or the darkness is long or short.

Sunrise/Sunset/Moonrise/Moonset

The sun and the moon appear to the human observer to rise in the East, crossing the sky in an arc until they set in the West. Sunrise is casually defined as when the observer from the earth looks at the bottom of the horizon and sees an edge of light rising slowly up from it into the horizon. Sunset is casually defined as when the sun disappears from full view to being unseen as it slips under the nadir to light up the other side of the world. It is now that it is dark enough to see the stars and the moon.

Moonrise and moonset are different than sunrise and sunset as the view of the moon is variable to the casual observer. So at times, the moon is visible in the sunlit day and some nights seem to have no moon. In other words, moonrise and moonset are not relative to a specific viewing locating on the earth on any given date. This is called the cycles of the moon.

The moon goes through certain phases depending on the sunlight that hits it. We know this because the markings on the moon don’t change as far as we can see. This is because the moon orbits the earth every 27.32 days on an eastward plane. The moon is held in its orbit by gravity in relation to the earth. This orbital time is called the sidereal period or the period that it takes the moon to circle the earth once. While it is circling the earth, the moon rotates quickly on its own axis so that the same side always faces the earth. The phases of the moon wax and wane about every four weeks in a cycle that takes 29.53 days or the synodic period.

The moon’s orbital radius is roughly equal to 30 diameters of the earth. Since the lunar orbit is slightly elliptical at about five degrees, the moon’s own diameter can vary at an angle of about six percent, plus or minus. So the moon’s path doesn’t always follow its elliptical exactly.

The cycle of the moon begins and ends with the full moon. If a full moon is seen twice in a calendar month, it is called a blue moon, which is relatively rare—hence, the old saying, “Once in a blue moon.”

The first two weeks of the cycle begins with the full moon, waxing gibbous, new moon, and waning crescent. The last two weeks of the cycle begins with the new moon, then the waning crescent, third quarter and the waning gibbous phase, which ends when the moon is once again full.

The Twilight Relationship

Twilight is that time where the moon is getting ready to rise and set in the sky that doesn’t have direct sunlight reflecting off the surface of the moon back to the earth. The light is naturally in the upper atmosphere of the earth toward the zenith of the horizon. This creates the silhouetted trees that mark the time in many locations, dependent upon weather conditions and location. Twilight can further be divided into civil, nautical, and astronomical twilight. Civil twilight is defined by the sun being six degrees below the horizon or six degrees past the nadir. Nautical twilight is defined at twelve degrees and astronomical twilight is defined as the sun being 18 degrees under the nadir. Civil twilight is easily seen where astronomical twilight is nearly imperceptible to the naked eye.

Transit of the Sun and the Moon

The sun and the moon are called celestial bodies. When either one crosses a certain longitudinal meridian, or line in the sky, the prime meridian, it is called transit or “transition” time. The sun crosses that line at noon, when the sun is at its highest point in the sky. The time before noon is called ante meridian and the time after the transition is called post meridian, or as we understand it, dividing the day into a.m. and p.m.

The moon transit is related to its phase in about six hour intervals. The new moon follows the sun transit, which is why we cannot see it and the night sky appears moonless. The first quarter transits at six hours after the sun, the full moon at twelve hours before and after, and the last quarter six hours before.