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DSST Astronomy practice questions

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Question 1Celestial mechanics

Kepler's second law states that a line joining a planet and the Sun sweeps out equal areas in equal times. Which physical quantity does this law most directly reflect being conserved?

  1. The planet's angular momentum about the Sun Correct answer.
  2. The planet's orbital period
  3. The planet's total mechanical energy
  4. The planet's linear momentum

Explanation

Equal areas in equal times is equivalent to a constant areal velocity, which follows from conservation of angular momentum in a central force field. Because the gravitational force is always directed along the line to the Sun, it exerts no torque about the Sun, so angular momentum stays fixed. Orbital energy is also conserved but does not produce the equal-area result.

Question 2Celestial mechanics

According to Kepler's third law, the square of a planet's orbital period is proportional to which quantity?

  1. The inverse of its orbital eccentricity
  2. The planet's mass times its semi-major axis
  3. The cube of its semi-major axis Correct answer.
  4. The square of its semi-major axis

Explanation

Kepler's third law relates period P and semi-major axis a by P squared proportional to a cubed for bodies orbiting the same central mass. In Newtonian form this becomes P squared equals 4 pi squared a cubed divided by G times the total mass. The law applies to elliptical and circular orbits alike when a is measured as the semi-major axis.

Question 3Celestial mechanics

A body in an elliptical orbit around the Sun moves fastest at which point in its orbit?

  1. Its speed is constant everywhere
  2. At aphelion
  3. At perihelion Correct answer.
  4. At the semi-minor axis endpoint

Explanation

Conservation of angular momentum requires the product of radius and transverse velocity to stay constant, so the body moves fastest where it is closest to the Sun. Perihelion is the closest point, so the orbital speed reaches its maximum there. At aphelion the body is farthest away and moves most slowly.

Question 4Celestial mechanics

Two satellites orbit Earth at the same altitude, one with twice the mass of the other. How do their orbital speeds compare?

  1. The more massive satellite moves twice as fast
  2. The more massive satellite moves faster by a factor of the square root of two
  3. The more massive satellite moves at half the speed
  4. Both satellites move at the same speed Correct answer.

Explanation

For a circular orbit, setting gravitational force equal to centripetal force gives v squared equals GM over r, where M is the mass of the central body. The satellite's own mass cancels out of the equation, so orbital speed depends only on the central mass and the orbital radius. Satellites at the same altitude therefore share the same speed regardless of their mass.

Question 5Celestial mechanics

A comet orbits the Sun with a perihelion distance of 1 AU and an aphelion distance of 9 AU. What is the semi-major axis of its orbit?

  1. 5 AU Correct answer.
  2. 10 AU
  3. 4 AU
  4. 8 AU

Explanation

The major axis of an ellipse equals the sum of the perihelion and aphelion distances, so here it is 1 AU plus 9 AU, or 10 AU. The semi-major axis is half of that sum, giving 5 AU. This averaging of the apsidal distances is a standard shortcut for elliptical orbits.

Question 6Celestial mechanics

Newton's law of universal gravitation states that the force between two masses is proportional to which combination?

  1. The product of the masses divided by the distance cubed
  2. The product of the masses divided by the distance squared Correct answer.
  3. The sum of the masses divided by the distance
  4. The difference of the masses divided by the distance squared

Explanation

The gravitational force equals G times the product of the two masses divided by the square of the separation. The inverse-square dependence on distance and the direct proportionality to each mass are the defining features of the law. This force is always attractive and acts along the line joining the two bodies.

Question 7Celestial mechanics

A spacecraft in a circular orbit fires its engine briefly to increase its speed in the direction of travel. What happens to its orbit?

  1. It becomes an ellipse with the burn point as perigee Correct answer.
  2. It spirals inward toward the central body
  3. Its orbital plane tilts by 90 degrees
  4. It becomes a smaller circular orbit at the same speed

Explanation

A prograde burn raises the speed above circular velocity at that radius, so the point of the burn becomes the closest point of a new elliptical orbit. The spacecraft then coasts outward to a higher apoapsis before returning. The orbit does not stay circular unless the speed change is applied in a way that preserves circular velocity.

Question 8Celestial mechanics

Escape velocity from a body of mass M and radius R is given by which expression?

  1. 2GM divided by R squared
  2. The square root of 2GM divided by R Correct answer.
  3. The square root of GM divided by R
  4. GM divided by R

Explanation

Escape velocity is found by setting kinetic energy equal to the magnitude of gravitational potential energy, giving one-half m v squared equals GMm over R. Solving yields v equal to the square root of 2GM over R. It exceeds circular orbital velocity at the same radius by a factor of the square root of two.

Question 9Celestial mechanics

Which statement correctly describes the relationship between a body's orbital period and its semi-major axis when orbiting a more massive central body?

  1. A larger semi-major axis gives a longer period Correct answer.
  2. A larger semi-major axis always gives a shorter period
  3. Period is independent of semi-major axis
  4. Period depends only on the orbiting body's mass

Explanation

Kepler's third law shows that period increases with semi-major axis, since P squared is proportional to a cubed. A body farther from the central mass travels a longer path at a lower average speed, so it takes more time to complete an orbit. This holds for any central mass, though the constant of proportionality changes with that mass.

Question 10Celestial mechanics

A planet's orbit has an eccentricity of 0. Which description fits this orbit?

  1. A parabola
  2. A hyperbola
  3. A perfect circle Correct answer.
  4. A highly elongated ellipse

Explanation

Eccentricity measures how much an ellipse deviates from a circle, ranging from 0 for a circle to values approaching 1 for very elongated ellipses. An eccentricity of exactly 0 means the foci coincide at the center, producing a circular orbit. Values of 1 or greater correspond to open parabolic or hyperbolic trajectories.

Question 11Celestial mechanics

An object is in a stable circular orbit at radius r around a central mass. If the central mass is quadrupled while the radius stays the same, the orbital speed changes by what factor?

  1. It quadruples
  2. It halves
  3. It doubles Correct answer.
  4. It stays the same

Explanation

Circular orbital speed is the square root of GM over r, so speed scales with the square root of the central mass. Quadrupling M multiplies the speed by the square root of four, which is two. The radius is unchanged, so no other factor enters.

Question 12Celestial mechanics

Which of the following best explains why a geostationary satellite must orbit at a specific altitude above Earth's equator?

  1. Because the satellite must escape Earth's gravity
  2. Because atmospheric drag is lowest at that altitude
  3. Because gravity is strongest at the equator
  4. Because the required orbital period must equal one sidereal day and the orbit must lie over the equator Correct answer.

Explanation

A geostationary satellite appears fixed over one point on Earth, which requires its orbital period to match Earth's rotation period of one sidereal day. Kepler's third law then fixes the radius, and the orbit must be equatorial and prograde so the satellite stays above the same longitude. The altitude is therefore determined by the period requirement, not by local gravity strength.

Question 13Celestial mechanics

A planet moves from perihelion to aphelion along its elliptical orbit. What happens to its gravitational potential energy and kinetic energy?

  1. Potential energy decreases while kinetic energy increases
  2. Potential energy increases while kinetic energy decreases Correct answer.
  3. Both potential and kinetic energy decrease
  4. Both potential and kinetic energy increase

Explanation

As the planet moves outward from perihelion to aphelion, its distance from the Sun increases, so gravitational potential energy rises toward zero from a negative value. Because total mechanical energy is conserved, the gain in potential energy is matched by a loss of kinetic energy, so the planet slows down. This trade-off is the energy counterpart of Kepler's equal-area law.

Question 14Celestial mechanics

Which quantity is conserved for a body moving in a central gravitational field and leads directly to the conclusion that orbits lie in a plane?

  1. The angular momentum vector Correct answer.
  2. The radial velocity
  3. The kinetic energy scalar
  4. The gravitational potential energy scalar

Explanation

In a central force field the torque about the center is zero, so the angular momentum vector is constant in both magnitude and direction. Because the position and velocity vectors must remain perpendicular to this fixed vector, the motion stays confined to a plane. Scalar energy quantities are conserved but do not constrain the orientation of the orbit.

Question 15Celestial mechanics

Two bodies orbit each other under their mutual gravity. In the two-body problem, each traces an ellipse about what point?

  1. A fixed point in space unrelated to the bodies
  2. The larger body's surface
  3. Their common center of mass Correct answer.
  4. The smaller body's center

Explanation

In the two-body problem both bodies orbit the system's center of mass, which moves at constant velocity if no external forces act. The more massive body traces a smaller ellipse and the less massive body a larger one, both about that shared point. When one mass greatly exceeds the other, the center of mass nearly coincides with the larger body.

Question 16Celestial mechanics

A satellite in a low circular orbit experiences slight atmospheric drag. Over many orbits, what happens to its orbital radius and speed?

  1. The radius increases and the speed decreases
  2. Both the radius and the speed increase
  3. The radius decreases and the speed increases Correct answer.
  4. Both the radius and the speed decrease

Explanation

Drag removes orbital energy, causing the satellite to settle into a lower orbit. In a circular orbit the speed is the square root of GM over r, so a smaller radius corresponds to a higher speed. This counterintuitive result occurs because the loss of potential energy exceeds the gain in kinetic energy.

Question 17Celestial mechanics

A spacecraft at Earth's surface is given exactly escape velocity. Ignoring air resistance and Earth's rotation, what is its speed far from Earth?

  1. Exactly escape velocity
  2. Zero Correct answer.
  3. Half of escape velocity
  4. Infinite

Explanation

Escape velocity is defined as the minimum launch speed for which total mechanical energy is exactly zero. As the spacecraft recedes, its kinetic energy is converted entirely into potential energy, and at infinite distance both approach zero. The spacecraft therefore arrives with essentially no residual speed, just barely escaping.

Question 18Celestial mechanics

A Hohmann transfer is used to move a spacecraft between two circular orbits. Which description matches this maneuver?

  1. Two impulsive burns, one to enter an elliptical transfer orbit and one to circularize at the target radius Correct answer.
  2. A single continuous thrust that spirals outward
  3. A retrograde burn that lowers periapsis below the surface
  4. A plane change performed at the same altitude

Explanation

A Hohmann transfer uses an elliptical transfer orbit tangent to both circular orbits, requiring a burn at the initial orbit to raise apoapsis and a second burn at the target to circularize. It is the most energy-efficient two-impulse transfer between coplanar circular orbits. The transfer ellipse has its periapsis at the inner orbit and apoapsis at the outer orbit.

Question 19Celestial mechanics

According to Kepler's first law, the Sun occupies which point of a planet's elliptical orbit?

  1. The end of the semi-minor axis
  2. One of the two foci Correct answer.
  3. The point of maximum distance from the planet
  4. The geometric center of the ellipse

Explanation

Kepler's first law states that planets move on ellipses with the Sun at one focus. The other focus is empty, and the center of the ellipse lies midway between the foci. For a circular orbit the two foci coincide at the center, which is a special case of the general statement.

Question 20Celestial mechanics

A planet orbits a star with a semi-major axis four times that of Earth's orbit. Assuming the star has the same mass as the Sun, what is the planet's orbital period in Earth years?

  1. 8 years Correct answer.
  2. 2 years
  3. 16 years
  4. 4 years

Explanation

Kepler's third law in solar units gives P squared equal to a cubed, with P in years and a in AU. For a equal to 4, a cubed is 64, and the square root of 64 is 8. The planet therefore completes one orbit in 8 Earth years.

Question 21Celestial mechanics

At what point in an elliptical orbit is the gravitational force on a planet strongest, and how does this relate to its acceleration?

  1. At perihelion, where the force is greatest but the acceleration is smallest
  2. At aphelion, where the force is smallest but the acceleration is greatest
  3. At aphelion, where the force and acceleration are both greatest
  4. At perihelion, where the force and acceleration are both greatest Correct answer.

Explanation

Gravitational force follows an inverse-square law, so it is strongest where the planet is closest to the Sun, at perihelion. Newton's second law makes acceleration proportional to force, so the acceleration is also greatest there. This is consistent with the planet moving fastest at perihelion.

Question 22Celestial mechanics

A body in a circular orbit at radius r has orbital speed v. What speed would it need to escape from that same orbital radius?

  1. v squared
  2. v divided by the square root of two
  3. 2v
  4. v multiplied by the square root of two Correct answer.

Explanation

Circular speed is the square root of GM over r, while escape speed is the square root of 2GM over r. The ratio of escape speed to circular speed is therefore the square root of two. A spacecraft in low orbit must increase its speed by about 41 percent to escape.

Question 23Celestial systems

Which statement correctly describes the relationship between a planet's orbital period and its average distance from the Sun, as summarized by Kepler's third law?

  1. The cube of the period is proportional to the square of the semi-major axis.
  2. The period is directly proportional to the semi-major axis.
  3. The period is inversely proportional to the square of the semi-major axis.
  4. The square of the period is proportional to the cube of the semi-major axis. Correct answer.

Explanation

Kepler's third law states that P squared equals a cubed when P is measured in years and a in astronomical units. This harmonic relationship links orbital size to orbital speed across all bodies orbiting the same mass. It applies to planets, moons, and binary stars alike.

Question 24Celestial systems

A planet is observed at superior conjunction. Where is it located relative to Earth and the Sun?

  1. Opposite the Sun in Earth's sky, rising at sunset.
  2. Between Earth and the Sun, on the near side of the Sun.
  3. At a 90-degree angle from the Sun as seen from Earth.
  4. On the far side of the Sun from Earth, aligned along the Sun-Earth line. Correct answer.

Explanation

Superior conjunction occurs when a planet lies beyond the Sun as seen from Earth, so the Sun sits between Earth and the planet. This configuration can only happen for planets whose orbits lie outside Earth's, such as Mars or Jupiter. Inner planets cannot reach superior conjunction geometry in the same sense because they pass between Earth and Sun at inferior conjunction.

Question 25Celestial systems

Why does the Moon show the same face to Earth at all times?

  1. Earth's gravity prevents the Moon from rotating at all.
  2. Its orbital period equals Earth's rotation period.
  3. Its rotation period equals its orbital period around Earth. Correct answer.
  4. Its rotation axis is exactly perpendicular to its orbital plane.

Explanation

The Moon is tidally locked: it completes one rotation on its axis in the same time it takes to orbit Earth. As a result, the same lunar hemisphere always points toward Earth. This synchronous rotation is common among large moons in the solar system.

Question 26Celestial systems

A solar eclipse can occur only near which lunar phase, and why?

  1. First quarter, because the Moon is at 90 degrees elongation.
  2. New Moon, because Earth's shadow falls on the Moon.
  3. Full Moon, because Earth passes between the Sun and the Moon.
  4. New Moon, because the Moon passes between Earth and the Sun. Correct answer.

Explanation

A solar eclipse happens when the Moon moves between Earth and the Sun and its shadow reaches Earth's surface. That alignment corresponds to the New Moon phase. Because the Moon's orbit is tilted about five degrees, not every New Moon produces an eclipse.

Question 27Celestial systems

An observer sees a star rising exactly in the east. Approximately how many hours later will that same star rise again?

  1. About 23 hours 56 minutes Correct answer.
  2. About 24 hours 4 minutes
  3. About 24 hours exactly
  4. About 12 hours

Explanation

A sidereal day is the time for Earth to rotate once relative to the distant stars, about 23 hours 56 minutes. Because Earth also moves along its orbit, a solar day is about four minutes longer. Stars therefore rise roughly four minutes earlier each successive night.

Question 28Celestial systems

Which of the following best explains why Earth experiences seasons?

  1. Earth's axial tilt changes the angle and duration of sunlight through the year. Correct answer.
  2. The Sun's luminosity varies with an annual cycle.
  3. The Moon's gravity tilts Earth's axis back and forth each year.
  4. Earth's distance from the Sun varies because its orbit is noticeably elliptical.

Explanation

Seasons arise because Earth's rotation axis is tilted about 23.5 degrees relative to its orbital plane. This tilt changes the solar elevation angle and daylight hours over the year, producing opposite seasons in the two hemispheres. Distance variations are too small and out of phase to be the main cause.

Question 29Celestial systems

What distinguishes a sidereal month from a synodic month?

  1. The sidereal month applies only to solar eclipses, while the synodic month applies only to lunar eclipses.
  2. The sidereal month is measured against the stars, while the synodic month is measured between successive identical lunar phases. Correct answer.
  3. The sidereal month includes leap days, while the synodic month does not.
  4. The synodic month is measured against the stars, while the sidereal month is measured between phases.

Explanation

A sidereal month is the Moon's orbital period relative to the fixed stars, about 27.3 days. A synodic month is the interval between successive New Moons (or any repeated phase), about 29.5 days, because Earth's motion forces the Moon to travel farther to realign with the Sun. The synodic month is therefore longer.

Question 30Celestial systems

A comet is observed to have a highly eccentric orbit. According to Kepler's second law, where does the comet move fastest?

  1. At perihelion, where it is closest to the Sun Correct answer.
  2. At the semi-minor axis endpoints, where the orbit is widest
  3. At aphelion, where it is farthest from the Sun
  4. Its speed is constant throughout the orbit

Explanation

Kepler's second law states that a line from the Sun to an orbiting body sweeps equal areas in equal times. To keep the swept area constant, the body must move fastest when it is nearest the Sun. For a comet, that maximum speed occurs at perihelion.

Question 31Celestial systems

Which planet in the solar system has the shortest orbital period around the Sun?

  1. Earth
  2. Venus
  3. Mercury Correct answer.
  4. Mars

Explanation

Mercury orbits closest to the Sun, at about 0.39 AU, and completes one revolution in roughly 88 Earth days. By Kepler's third law, smaller semi-major axes correspond to shorter periods. Among the listed planets, Mercury therefore has the fastest orbit.

Question 32Celestial systems

What is the primary reason Venus appears brightest when observed near its greatest illuminated phase rather than at inferior conjunction?

  1. Earth's atmosphere magnifies Venus only near elongation.
  2. At inferior conjunction the unlit hemisphere faces Earth, while near greatest elongation a large illuminated fraction is visible. Correct answer.
  3. Venus emits its own light, which is blocked by the Sun at conjunction.
  4. At inferior conjunction Venus is farther from Earth than at any other point in its orbit.

Explanation

Venus shows phases like the Moon. At inferior conjunction it lies between Earth and the Sun, so its dark side faces us and it is lost in solar glare. Near greatest elongation a substantial illuminated fraction is visible at a favorable angle, making it appear very bright.

Question 33Celestial systems

The ecliptic is best defined as which of the following?

  1. The celestial equator projected onto the sky
  2. The line connecting the celestial poles
  3. The apparent annual path of the Sun against the background stars Correct answer.
  4. The path of the Moon across the sky during one night

Explanation

The ecliptic is the great circle traced by the Sun's apparent yearly motion against the constellations. It reflects Earth's orbital plane projected onto the celestial sphere. The Moon and planets appear near the ecliptic because their orbital planes are close to Earth's.

Question 34Celestial systems

Why do the planets appear to wander relative to the fixed stars over weeks and months?

  1. They orbit the Sun at different rates, so their positions shift against the stellar background. Correct answer.
  2. They physically move through the constellations because the stars orbit them.
  3. They are attached to spheres that rotate at variable speeds.
  4. They are much closer than stars, so atmospheric refraction displaces them differently.

Explanation

Planets are solar system bodies moving in their own orbits, so their positions change against the distant stars over time. The word planet derives from the Greek for wanderer. Their apparent retrograde loops arise from the combination of Earth's and the planet's orbital motions.

Question 35Celestial systems

What causes the apparent retrograde motion of Mars as seen from Earth?

  1. Earth overtakes Mars on the inside of its orbit, so Mars appears to move backward against the stars. Correct answer.
  2. Mars reverses its actual direction of revolution around the Sun.
  3. Earth's rotation briefly reverses, altering the apparent direction.
  4. Mars's axial tilt changes so its surface appears to shift.

Explanation

Retrograde motion is a geometric effect. Earth, moving faster in a smaller orbit, periodically laps Mars; during the overtaking, Mars appears to drift westward against the background stars before resuming eastward motion. Mars never actually reverses its orbital direction.

Question 36Celestial systems

At what phase is the Moon highest in the sky at local midnight for an observer at mid-northern latitudes?

  1. Full Moon Correct answer.
  2. First quarter
  3. Waxing crescent
  4. New Moon

Explanation

At Full Moon the Moon is opposite the Sun, so it rises near sunset and culminates near local midnight. Its high altitude at midnight reflects this opposition geometry. New Moon, by contrast, is near the Sun and is highest around noon.

Question 37Celestial systems

Which statement correctly describes the celestial equator as seen from Earth's North Pole?

  1. It coincides with the horizon. Correct answer.
  2. It lies below the horizon and is never visible.
  3. It is inclined 45 degrees to the horizon.
  4. It passes through the zenith.

Explanation

At the North Pole, the zenith points at the north celestial pole. The celestial equator, 90 degrees from that pole, therefore lies exactly along the horizon. Stars north of the equator circle the sky parallel to the horizon and never set.

Question 38Celestial systems

What is the primary cause of the tides on Earth?

  1. The Moon's magnetic field interacting with seawater
  2. The gravitational pull of the Sun alone
  3. Earth's rotation dragging the oceans eastward
  4. Differences in the Moon's gravitational pull across Earth's diameter Correct answer.

Explanation

Tides arise because the Moon's gravity pulls more strongly on the near side of Earth than on the far side, creating a tidal bulge on both sides. The Sun contributes as well but is less influential because of its greater distance. Earth's rotation carries observers through these bulges, producing two high tides per day.

Question 39Celestial systems

A star is observed to rise at 9 p.m. tonight. Approximately when will it rise one month from now?

  1. About 7 p.m. Correct answer.
  2. About 9 p.m.
  3. About 11 p.m.
  4. About 3 a.m.

Explanation

Stars rise about four minutes earlier each night because of Earth's orbital motion around the Sun. Over roughly 30 days this accumulates to about two hours earlier. A star rising at 9 p.m. tonight would therefore rise near 7 p.m. a month later.

Question 40Celestial systems

Which observation provided strong evidence that Earth orbits the Sun rather than remaining stationary?

  1. The daily rising and setting of the Sun
  2. The phases of Venus alone
  3. The retrograde motion of Mars alone
  4. Stellar parallax, the apparent shift of nearby stars against distant ones over a year Correct answer.

Explanation

Stellar parallax is the tiny annual shift of nearby stars relative to more distant background stars, caused by Earth's changing vantage point. Detected in 1838, it directly demonstrated Earth's orbital motion. Daily solar motion and phases can be explained by other geometries, but parallax requires Earth to move.

Question 41Celestial systems

What is the significance of the synodic period of a planet?

  1. It is the time between successive oppositions of Earth and the Sun.
  2. It is the planet's rotation period measured at the equator.
  3. It is the interval between successive identical configurations as seen from Earth. Correct answer.
  4. It is the time the planet takes to orbit the Sun relative to the stars.

Explanation

The synodic period is the time for a planet to return to the same configuration relative to Earth and the Sun, such as from one opposition to the next. It differs from the sidereal period because Earth is also moving. For Mars, the synodic period is about 780 days.

Question 42Celestial systems

Why does the Moon appear slightly larger when near the horizon than when high in the sky?

  1. It is an illusion produced by the brain's interpretation of the skyline context. Correct answer.
  2. Earth's atmosphere physically magnifies the Moon near the horizon.
  3. Earth's rotation stretches the lunar image at low altitude.
  4. The Moon is actually thousands of kilometers closer when rising.

Explanation

The Moon's angular size is essentially unchanged from horizon to zenith, as photographs confirm. The perceived enlargement is a psychological effect linked to how the brain judges distance using foreground cues. Atmospheric refraction slightly flattens the Moon's shape but does not magnify it.

Question 43Celestial systems

Which configuration places an inferior planet at its greatest angular separation from the Sun as seen from Earth?

  1. Quadrature
  2. Superior conjunction
  3. Opposition
  4. Greatest elongation Correct answer.

Explanation

Greatest elongation is the maximum angular distance an inferior planet reaches from the Sun in our sky, occurring when the planet-Earth-Sun angle is 90 degrees. It is the best time to observe Mercury or Venus because they are farthest from solar glare. Opposition and quadrature apply to superior planets.

Question 44Celestial systems

Precession of Earth's axis primarily affects which of the following over a 26,000-year cycle?

  1. The orbital period of Earth around the Sun
  2. The length of Earth's day
  3. The Moon's orbital inclination
  4. The identity of the pole star and the timing of the equinoxes along the orbit Correct answer.

Explanation

Earth's axis slowly wobbles like a spinning top, completing a circuit in about 26,000 years. This precession changes which star lies near the north celestial pole and shifts the equinox positions relative to the orbit. It does not significantly alter day length or Earth's orbital period.

Question 45Astronomical instruments

A telescope's aperture diameter is doubled while all other properties stay the same. How does the theoretical diffraction-limited angular resolution change?

  1. It improves by a factor of four
  2. It stays exactly the same
  3. It improves by a factor of two Correct answer.
  4. It worsens by a factor of two

Explanation

The diffraction limit scales as 1.22λ/D, so resolution is inversely proportional to aperture diameter. Doubling D halves the smallest resolvable angle, meaning resolution improves by a factor of two. The improvement is linear in diameter, not squared.

Question 46Astronomical instruments

Which property of a telescope is most directly increased by using a larger primary mirror or lens?

  1. Light-gathering power Correct answer.
  2. Focal ratio
  3. Eyepiece magnification
  4. Chromatic aberration

Explanation

Light-gathering power is proportional to the area of the aperture, which grows with the square of the diameter. A larger primary therefore collects more photons per unit time. Focal ratio and magnification depend on focal lengths and eyepiece choice, not aperture area alone.

Question 47Astronomical instruments

A refracting telescope uses an objective with focal length 1200 mm and an eyepiece with focal length 20 mm. What is the angular magnification?

  1. 0.017×
  2. 24×
  3. 120×
  4. 60× Correct answer.

Explanation

Angular magnification equals objective focal length divided by eyepiece focal length. Here 1200 mm divided by 20 mm gives 60×. The result is a dimensionless ratio, not a length.

Question 48Astronomical instruments

What is the primary purpose of a telescope's equatorial mount?

  1. To eliminate atmospheric turbulence from the image
  2. To track stars by rotating about an axis parallel to Earth's rotation axis Correct answer.
  3. To increase the aperture's light-collecting area
  4. To correct chromatic aberration in the objective

Explanation

An equatorial mount aligns one axis with Earth's polar axis, so a single slow rotation compensates for Earth's turning and keeps objects in view. It is a mechanical tracking solution, not an optical one. It does not change aperture or correct lens defects.

Question 49Astronomical instruments

Which detector technology replaced photographic plates in modern professional astronomy because of its high quantum efficiency and linear response?

  1. Human-eye sketching
  2. Charge-coupled devices Correct answer.
  3. Selenium photometers
  4. Photographic emulsions

Explanation

Charge-coupled devices convert incoming photons to electronic charge with quantum efficiencies often above 80 percent and a linear response to light level. Photographic plates have low efficiency and a nonlinear response. This made CCDs the standard imaging detector.

Question 50Astronomical instruments

Why are radio telescopes built with much larger dishes than optical telescopes?

  1. Radio sources are always brighter than optical sources
  2. Radio waves cannot pass through the atmosphere
  3. Radio dishes must be kept cool to detect photons
  4. Radio wavelengths are long, so a large aperture is needed for comparable angular resolution Correct answer.

Explanation

Angular resolution depends on wavelength divided by aperture diameter. Because radio wavelengths are thousands of times longer than visible light, a much larger dish is required to achieve similar resolution. This is why interferometry is also widely used at radio wavelengths.