Introduction to Astronomy

Introduction to Astronomy

Night sky basics for beginners in Guyana: what astronomy is, how big the universe is, how to measure the sky with your hand, and the first constellations to find from about 6-7° N.

Local Sky Sphere · Tonight's Top 3 Highlights · First Night Checklist

All times are Guyana time (America/Guyana, UTC-4, no daylight saving). Sky positions are for Georgetown (6.8° N) and change very little anywhere in Guyana.

Astronomy is not astrology

Astronomy is the scientific study of everything beyond Earth's atmosphere: the Moon, planets, stars, galaxies and the universe itself. Astronomers observe, propose an explanation, make a prediction and test it with new observations. Physics does the explaining: gravity holds planets in orbit, and nuclear fusion makes stars shine.

Astrology is the belief that the positions of the Sun, Moon and planets shape your personality or predict your future. Its claims are not supported by scientific evidence. The two share some names, such as the zodiac constellations, but only astronomy tests its ideas against measurements.

How big is the universe?

Space is so large that we change units as we move outward.

Beyond the Solar System we use the light-year, the distance light travels in a year. Light moves at 299,792,458 m/s, so a light-year is about 9.46 × 10¹² km.

The universe is only 13.8 billion years old, so how can we see 46.5 billion light-years? Space kept expanding while the light travelled, so the places that sent the oldest light are now much farther away.

Step out from the Moon to the edge of the observable universe: each rung's distance, how long its light takes to reach you, and where it would sit in the walking scale model where 1 AU is 1 metre.

| Unit | Size | Used for | |---|---|---| | Kilometre (km) | 1,000 m | Earth and the Moon | | Astronomical unit (AU) | 149,597,871 km (about 1.496 × 10⁸ km) | Planet orbits | | Light-year (ly) | 9.461 × 10¹² km = 63,241 AU | Distances to stars | | Parsec (pc) | 3.086 × 10¹³ km = 3.26 ly | Stars; kiloparsecs (kpc) in our galaxy, megaparsecs (Mpc) for other galaxies |

As Earth orbits the Sun, a nearby star seems to shift a little against far more distant stars. This shift is parallax. A star 1 parsec away has a parallax of 1 arcsecond. Sirius has a parallax of 0.379 arcseconds, so it is 2.64 parsecs (8.6 light-years) away.

Time scales are just as large. Earth is about 4.54 billion years old, and life goes back about 3.8 billion years. The first stars formed roughly 100 to 200 million years after the Big Bang. Stars live from millions to trillions of years: the more massive the star, the shorter its life.

Measuring the sky in angles

On the sky we measure angles, not distances. A circle is 360 degrees (°), 1° is 60 arcminutes (′), and 1′ is 60 arcseconds (″).

For small angles, angular size (in radians) = real size ÷ distance. Multiply by 57.3 to get degrees. The Moon is 3,474 km wide and 384,400 km away: 3,474 ÷ 384,400 = 0.0090 radians, about 0.5°. The Sun is about 400 times wider and about 400 times farther away, so it also looks about 0.5° across.

| Object or instrument | Size or sharpest detail | |---|---| | Moon and Sun | about 0.5° (30′) | | Planets | a few arcseconds up to about 1′ (Venus at its closest, 66″) | | Star disks | milliarcseconds, measured only by linked telescopes (interferometry) | | Human eye | about 1′ | | 7x50 binoculars | about 10″ in practice (the eye's 1′ divided by 7; the 50 mm lenses alone could reach about 2″) | | Small telescope | about 1″ | | Hubble Space Telescope | about 0.1″ |

Pick the Moon, the Sun or a planet and see how wide it looks today, worked out with the lesson's rule (size ÷ distance), and whether your eye, binoculars or a small telescope shows it as a disc.

Your hand is a ruler. At arm's length a fingertip covers about 1° and a closed fist about 10°. The full Moon is about half your little finger wide.

Sun safety: never look at the Sun with your eyes, binoculars, a telescope or a camera unless a proper solar filter covers the front of the optics. Use your hand ruler on the Moon and stars only.

Your fingertip, your fist, the full Moon, Orion's Belt and the Southern Cross compared at arm's length.

What is up there

Explore the planets on the Solar System page, learn when each is easy to see in Solar System Overview, and find clusters and nebulae in Deep Sky Objects Introduction.

Your sky from Guyana

Near the equator you see almost the whole sky over a year. From Georgetown the north celestial pole is only 6.8° above the northern horizon. Only stars within about 7° of the south celestial pole never rise.

The sky turns east to west about 15° an hour as Earth spins. Stars on the celestial equator rise due east and set due west. Each star rises about 4 minutes earlier each night, about 2 hours earlier each month, so each season has its own evening constellations.

The sky as a dome over Georgetown, Lethem and London: how high the celestial pole stands, which stars never set and which never rise.

The whole sky over Georgetown through one moonless night, sunset to sunrise on 10-11 October 2026: everything rises in the east and sets in the west, Saturn is up all night, and Mars and Jupiter rise after midnight.

Weather decides more nights than the calendar. On the coast the dry seasons are August to October and February to March, and the wet seasons mid-April to the end of July and mid-November to the end of January. Southern Guyana has one wet season, about April to August.

First constellations and asterisms

A constellation is one of 88 official areas of the sky. An asterism is a well-known star pattern that is not one of them, such as the Big Dipper or the Summer Triangle. Stars in a pattern are usually not close together in space; they only line up from where we stand.

| Pattern | Best evenings (around 20:00) | Where to look from Georgetown | |---|---|---| | Orion and its Belt | December to April | Rises due east; the Belt is nearly overhead in mid-February (about 82°) | | Sirius (Canis Major) | January to April | South, up to 66° | | Canopus (Carina) | January to April | Low in the south, up to about 30° | | Pleiades and Aldebaran (Taurus) | November to February | North-east, then high in the north; Pleiades up to 73° | | Big Dipper (Ursa Major) | April to June | Low in the north, 35-48° at best | | Southern Cross (Crux) | April to July | Due south, only 20-26° up, highest in May and June | | The Pointers (Alpha and Beta Centauri) | May to August | South, about 22° up, left of the Cross | | Scorpius and Antares | June to September | South-east, then south-west; Antares up to 57° | | Summer Triangle (Vega, Altair, Deneb) | August to October | Altair almost overhead (88°); Vega and Deneb 51-58° up in the north |

The table's patterns on the evening sky at 20:00 on the 15th of each month, with how high each one is and how high it can climb.

Orion is the best first constellation. Find three similar stars in a straight line about 3° long: Orion's Belt. Red Betelgeuse marks one shoulder and blue-white Rigel one foot. The misty patch in the Sword, the short line of stars beside the Belt, is the Orion Nebula. Follow the Belt about 23° one way to Sirius, the brightest star in the night sky, and 23° the other way to orange Aldebaran and the Pleiades. Canopus, the second brightest star, sits low in the south.

The Southern Cross fits behind your fist at arm's length and never climbs above about 26° here, so you need a clear southern horizon. The two bright Pointers beside it lead to it. The brighter one, Alpha Centauri, is the nearest star system to the Sun; Proxima Centauri belongs to it. Beware the False Cross, a larger, dimmer cross that rises earlier.

Scorpius is a hook of stars with red Antares at its heart. Its tail points to the Teapot of Sagittarius and the bright core of the Milky Way. See Milky Way Season Near the Equator and the Milky Way Season Planner for the best months.

The Big Dipper stays low from Guyana. The two stars at the end of its bowl point to Polaris, the North Star, but Polaris is only about 7° up here, and trees or haze often hide it. Finding Objects in the Night Sky Without Polaris shows other ways to find your way.

What you can see tonight

  1. Check clouds and darkness with the Tonight Observing Score or Stargazing Tonight in Georgetown.
  2. Open Tonight's Top 3 Highlights for a short list of what is worth seeing.
  3. Use Planets Visible Tonight to see which planets are up and when.
  4. Open the Local Sky Sphere and drag to look around. It marks the Moon, planets, constellations and deep-sky objects where they are now; tap a marker for its altitude and direction.
  5. Check Sky Events for meteor showers, eclipses and Moon-planet pairings.

Guyana's sky tonight from Georgetown or Lethem, from 18:00 to 05:00: drag the time and look for the four compass points, the Moon and the planets.

The Moon is the easiest target: The Moon and Lunar Phenomena explains its phases, and Why Seasons Happen explains why the Sun's path shifts through the year. City lights hide faint stars and the Milky Way, so use the Bortle Map (Dark Sky Finder) to find a darker site and read What Is the Bortle Scale?.

How bright tonight's planets and the first stars to learn are, and the faintest star your eye can catch from Georgetown and from Lethem.

How we know

People tracked the sky for calendars, seasons and navigation long before telescopes. Greek scholars used geometry to measure Earth's size, Ptolemy placed Earth at the centre, and astronomers of the Islamic Golden Age preserved and improved that work. Copernicus moved the Sun to the centre, Galileo turned a telescope on the sky, Kepler described planetary orbits, and Newton explained them with gravity. Herschel's deep-sky discoveries, spectroscopy and photography followed, then relativity, the expanding universe, the nuclear physics of starlight, radio astronomy and today's space telescopes and planetary probes. The method never changed: observe, explain, predict and test, then let others check the work through peer review and shared data. Every measurement has limits, so good science states its uncertainty, and anyone can help through citizen science.

Try it: activities for home or class

Before your first night out, use the First Night Checklist.

Quick quiz

How many astronomical units are in one light-year?

About 63,241 AU. (206,265 AU is one parsec, 3.26 light-years.)

Why do the Moon and Sun look about the same size?

The Sun is about 400 times wider and about 400 times farther away, so both cover about 0.5°.

How high does the Southern Cross climb from Georgetown?

Only about 20-26° above the southern horizon, highest on May and June evenings.

A constellation you saw at 22:00 this month will be in the same place at about what time next month?

About 20:00, because stars rise about 4 minutes earlier each night.

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