Since ancient times, the stars in the night sky have been connected into patterns called constellations. The constellations visible in each season change because of Earth’s revolution.
Basic Knowledge
The International Astronomical Union (IAU) has officially defined 88 constellations. About half can be seen from Japan.
Representative seasonal constellations and first-magnitude stars (triangles):
① Spring Triangle: Arcturus (Boötes)・Spica (Virgo)・Denebola (Leo).
② Summer Triangle: Vega (Lyra)・Altair (Aquila)・Deneb (Cygnus).
③ Autumn Square (Pegasus): A quadrilateral that serves as a landmark, although it contains few first-magnitude stars.
④ Winter Triangle: Betelgeuse (Orion)・Sirius (Canis Major)・Procyon (Canis Minor).
Even for the same stars, the direction in which they are visible at night changes with the seasons because Earth revolves around the Sun. This is called annual motion, and constellations shift approximately 30 degrees westward in one month.
Constellation（A region of the sky formed by connecting stars into a shape. There are 88）
Annual motion（The seasonal shift in when constellations are visible due to Earth’s revolution）
Spring Triangle（Arcturus・Spica・Denebola）
Summer Triangle（Vega・Altair・Deneb）
Winter Triangle（Betelgeuse・Sirius・Procyon）
Zodiac constellations（The 12 constellations along the Sun’s path (ecliptic). Constellations of astrology）
Further Exploration (Background・Significance)
When observed at the same time, constellations appear to move approximately 30 degrees, or about 1 degree per day, westward in one month. This is annual motion; in one year, they move 360 degrees and return to their original positions.
In ancient Egypt, the time when Sirius appeared on the eastern horizon just before sunrise coincided with the flooding of the Nile River, so it was used to create calendars.
The zodiac constellations (Aries・Taurus・Gemini・Cancer・Leo・Virgo・Libra・Scorpius・Sagittarius・Capricornus・Aquarius・Pisces) lie along the Sun’s path (the ecliptic) over one year and are the origin of astrology. In fact, because of the precession of Earth’s axis, the Sun is currently in a constellation approximately one constellation away from the one traditionally associated with each birth month.
In cities, light pollution makes stars difficult to see, and the dark-sky movement to preserve the night sky is spreading.
- There are 88 constellations (officially recognized by the IAU)
- Spring Triangle = Arcturus・Spica・Denebola
- Summer Triangle = Vega・Altair・Deneb
- Winter Triangle = Betelgeuse・Sirius・Procyon
- Constellations move approximately 30 degrees westward in one month (annual motion)
- Zodiac constellations = the Sun’s path
- Sirius is the brightest star in the entire sky
Points to Note (Easily Confused)
① There are spring, summer, and winter triangles, but autumn has a quadrilateral (Pegasus). ② Constellations move 30 degrees (=360÷12) in one month and approximately 1 degree westward in one day. ③ Diurnal motion (east→west in one day, caused by rotation) and annual motion (30 degrees westward per year, caused by revolution) are different. ④ The constellations of astrology and their positions in modern astronomy differ because of precession.
Practice
- State all three first-magnitude stars that form the Summer Triangle.
- When observed at the same time, approximately how many degrees westward do constellations move in one month?
- What is the path along which the Sun moves called?