Four seasons—spring, summer, fall, and winter.
The Sun’s rising point and setting point and the length of the day and night changes depending on the season.
When the Sun sets and night arrives, we are able to observe numerous stars.
The changing seasons also change the stars we are able to observe.
Let’s learn more about what brings about these changes.
Revolution and Annual Motion
The vast Earth we live on makes a full circle around the Sun, from west to east, in one year.
This is called the revolution of the Earth.
There are 365 days in a year and a full circle is 360 degrees,
which means that the Earth only moves around the Sun one degree per day.
From our perspective on Earth, it looks like the Sun is moving.
The apparent motion of the Sun,
as it appears to move one degree from west to east due to the revolution of the Earth, is called the “annual motion of the Sun.”
The ecliptic is the path of the Sun’s annual motion along the celestial sphere.
The ecliptic is tilted at a 23.5 degree angle compared to the equator of the celestial sphere.
The point on the ecliptic with the highest declination is called the summer solstice point,
and the point with the lowest declination is called the winter solstice point.
The Sun’s annual motion also affects the seasonal locations of the Earth and Sun.
The Earth’s revolution around the Sun forms an oval that is almost a circle.
On the Earth’s orbit of revolution, the point closest to the Sun is called the perihelion,
and the point farthest from the Sun is called the aphelion.
In the Northern Hemisphere, the aphelion is in summer and the perihelion is in winter.
When the Northern Hemisphere is experiencing summer, the point where the Sun is visible is called the summer solstice point,
and when the Northern Hemisphere is experiencing winter, the point where the Sun is visible is called the winter solstice point.
The equator of the celestial sphere and the ecliptic meet at two points.
The point going up from the Southern Hemisphere to the Northern Hemisphere is called the vernal equinox,
and the point going down from the Northern Hemisphere to the Southern Hemisphere is called the autumnal equinox.
In summary, during the spring, summer, fall, and winter, the Sun is located at the vernal equinox,
summer solstice point, autumnal equinox, and winter solstice point, respectively.
Now, let’s locate the Sun in each season using specific coordinates called right ascension and declination.
Right ascension is the angle measured counter clock-wise in hour circles starting at the vernal equinox
and moving along the equator of the celestial sphere up to the celestial body.
15˚ is converted into 1 hour and expressed in a range of 0h to 24h.
Declination is the angular distance between the celestial body and the celestial equator in hour circles
and is expressed in (+) for north of the celestial equator and (-) for south of the celestial equator.
The declination ranges between -90˚ and +90˚.
The vernal equinox has a right ascension and declination of 0h and 0˚, the summer solstice 6h and +23.5˚,
the autumnal equinox 12h and 0˚, and the winter solstice 18h and -23.5˚.
Because of the Earth’s revolution, the Sun’s annual motion is observed in the direction of vernal equinox,
summer solstice point, autumnal equinox, and winter solstice point.
The Earth’s revolution not only affects the Sun’s annual motion but also brings about changes in seasonal constellations as well as the actual seasons.
Our zodiac signs are based on the date we were born.
The 12 zodiac signs refer to the 12 constellations near the ecliptic.
Why do we see different constellations depending on the season?
Constellations in the direction of the Sun are blocked from view by strong sunlight, while constellations that are in the opposite direction of the Sun are in clear view.
This is why the constellations change depending on the direction the Earth faces as it revolves around the Sun.
Let’s look at a specific example.
In Korea during the spring, summer, fall, and winter, the right ascension of the Sun is 0h at vernal equinox,
6h at the summer solstice point, 12h at the autumnal equinox, and 18h at the winter solstice point.
This means that the observable constellations in the spring are Virgo and Leo,
located on the opposite side of the autumnal equinox at 12h.
in the summer, Archer and Scorpio are visible on the opposite side of the winter solstice point at 18h winter.
In the fall, we see Pisces and Aquarius, and in the winter, Gemini and Taurus are visible
Stars also have annual motion.
What’s different is that stars move from east to west, unlike the Earth’s revolution and the Sun’s annual motion.
While the star’s diurnal motion and annual motion both occur in a clockwise direction,
the diurnal motion has a cycle of one day, and the annual motion is based on one year,
which means that diurnal motion has an apparent motion of 15 degrees an hour,
and annual motion has an apparent motion of one degree a day.
Now let’s take a look at the changes in season.
Seasons change because the Earth revolves around the Sun with a rotation axis tilted at an angle of 23.5 degrees against the orbital plane of the revolving body.
As the Earth revolves around the Sun,
the hemisphere with the greater solar radiant energy per unit area experiences summer, while the other hemisphere—the one with lower solar radiant energy per unit—experiences winter.
As we’ve already discussed,
the Northern Hemisphere experiences summer when the Earth is located at the aphelion and winter when the Earth is located at the perihelion.
Seasonal changes can also be explained by meridian altitude.
So what exactly is meridian altitude?
Meridian altitude refers to various altitudes, including: the altitudes of celestial bodies when they enter the southern meridian during diurnal motion;
the altitude of celestial bodies when they move south; the altitudes of celestial bodies when they are located due south in the sky;
the altitudes of celestial bodies when they are located at the highest point of the southern surface
How is the meridian altitude of the Sun calculated?
The meridian altitude of the Sun is 90-Φ+δ in the Northern Hemisphere when the observer’s latitude is Φ and the Sun’s declination is δ.
Keeping this in mind, let’s calculate the meridian altitude of the Sun for each season.
Assuming that the observer is located at a latitude of 37 degrees north of the equator,
the declination of vernal equinox, summer solstice point, autumnal equinox, and winter solstice point is 0 degrees, +23.5 degrees, 0 degrees, and -23.5 degrees, respectively.
This means that the meridian altitude of the Sun in spring, summer, fall, and winter is 53 degrees, 76.5 degrees, 53 degrees, and 29.5 degrees, respectively.
In Korea, the meridian altitude of the Sun is the highest during summer solstice and the lowest during winter solstice.
This means that the higher the meridian altitude, the larger the amount of solar radiant energy per unit area.
So, solar radiation increases, which then causes temperatures to rise.
The season changes the location of the Sun’s rise and fall and the length of day and night.
Korea has longer days in the summer, longer nights in the winter, and days and nights that are similar in length in the spring and fall.
The Sun is located at the celestial equator during vernal equinox and autumnal equinox, and rises in the due east and sets in the due west.
However, during the summer solstice when the Sun’s declination is +23.5˚,
the diurnal arc appears above the equator, which means that the Sun rises in the northeast and sets in the northwest with a higher meridian altitude, longer Sunshine duration, and increased solar radiation.
During the winter solstice when the Sun’s declination is -23.5˚,
the diurnal arc appears below the equator, which means that the Sun rises in the southeast and sets in the southwest with a lower meridian altitude, shorter Sunshine duration, and decreased solar radiation.
The Earth moves, but we are often unaware of its effects.
It’s fascinating to know that the Earth’s movement generates various phenomena that affect our daily lives.
Before you go to bed tonight, check and see what stars you can spot in the night time sky!
Series of why
