Today’s Lecture

• How to find astronomical objects– Constellations, coordinates, precession

• Motion of stars on the sky

• First homework is due at the start of class on Monday.

How does one locate an object on the night sky?

1. By drawing imaginary patterns on the sky (the constellations) and then locating the object relative to the stars in the constellations.

2. By drawing an imaginary coordinate system on the sky, then specifying the objects coordinates.

Eighty-eight constellations cover the entire sky.

• 6000 stars visible to unaided eye (only half are above the horizon).

• 88 semi-rectangular groups of stars called constellations.

• Some stars in the constellations are quite close while others are very far away.

Finding M51

Finding M51

Finding M51

We use angles to denote the positions and apparent sizes of objects in the sky.

Your hand at arm’s length is about 10 degrees wide

Your thumb at arm’s length is about 2 degrees wide

Coordinate system

Coordinates are

Latitude = degrees North or South of the equator

Longitude = degrees East or West of the “Prime meridian”

Prime meridian is historically defined as longitude of the Royal Observatory in Greenwich, England

Sky coordinate system

Introduce the ‘celestial sphere’

This is an imaginary sphere drawn in space with the earth at its center.

We align the sphere with the Earth.

Coordinates are:

Declination = degrees North or South of the equator.

Right ascension = hours (or degrees) East of the “Vernal equinox”.

Vernal equinox is defined as the position of the Sun on the first day of spring. Note it is a point on the sky, not the earth.

Precession

• If you spin a top, its very hard to get it to spin exactly straight – usually it wobbles around in a circle

• The spinning Earth wobbles in exactly the same way – this is called precession

Precession of the Earth

Precession causes celestial coordinates to change slowly with time. When observing, one must have coordinates for the correct epoch.

Motion of stars on the

sky

The rotation of the Earth causes the stars to appear to move on the sky.

Angular Measure for Small Angles

1º = 60 arcminutes = 60′

1′ = 60 arcseconds = 60″

e.g., On January 1, 2004, the planet Saturn had an angular diameter of 19.7″ as viewed

from Earth.

Coordinates• Right Ascension hours:minutes:seconds

– There are 24 hours in 360°.• Declination degrees:arcminutes:arcsecond

– 1º = 60′, 1′ = 60″

• Polaris: RA = 02h31m49s, Dec = 89°15′51″ (J2000)

• Sirius: RA = 06:45:09, -16:42:58 (J2000)• Sirius lies in Southern hemisphere, but is visible from Iowa.

• Stars visible from Iowa have Dec > 41.7° - 90° = -48.3°.

Hour angle• Meridian is the half-circle running from one pole to the other through a

point directly overhead. During the night, a star will reach its highest elevation above the horizon when it passes through the meridian (‘transits’).

• Sidereal time is time kept relative to the stars (rather than the Sun). Local sidereal time = 0h when a star with RA = 0h crosses the meridian (like noon = Sun crossing the meridian).

• RA uses time units to make it easy to compute the position of a star on the sky at any time of night.

• A star transits when the sidereal time equals its right ascension.• Hour angle = Local sidereal time - right ascension = position of star

relative to meridian.• RA determines what time of year an object is visible. On March 21,

objects with RA = 12h 00m transit at midnight.• Best to observe objects when they are high in the sky to minimize the

amount of air you need to look through.

Coordinates• Which objects would be reasonable choices for 29:62

observing projects? (All in J2000)

• 12 29 07, 02 03 09 (3C273)• 05 23 35, -69 45 22 (LMC)• 00 42 44, 41 16 08 (M31)• 02 31 49, 89 15 51 (?)

• Object Seasonal Observability (Rigel is near Kitt Peak)• Coordinate converter

Review Questions• What is the angular size of the Moon? How

does it compare to the angular size of the Sun?

• Why are the coordinates (RA, Dec) for a star measured in 1950 different from the coordinates measured for the same star in 2000, even if the star is not moving in space?

• Would it be prudent to do an observing project on M82 for this class?