astrophysics.txt

impulse
    Change in velocity.

Hoffmann Transfer Orbit
    https://www.youtube.com/watch?v=bNox3jUG31Y

    The most efficient way to get from one
    orbit to another.

    All it needs are 2 impulses.
    
    That will get you from Earth's orbit and
    eventually into Jupiter's orbit.

    It's not the fastest way but it's the most
    efficient way.

particle horizon
    [cosmological horizon]

    The maximum distance from which light from
    particles could have traveled to the
    observer in the age of the universe.
    
    It represents the boundary between the
    observable and the unobservable regions of
    the universe, so its distance at the
    present epoch defines the size of the
    observable universe.

hubble horizon
    [cosmological horizon]

    A conceptual horizon defining the boundary
    between particles that are moving slower
    and faster than the speed of light
    relative to an observer at one given time.
    
    Note that this does not mean the particle
    is unobservable, the light from the past
    is reaching and will continue to reach the
    observer for a while.
    
    Also, more importantly, in the current
    expansion model e.g., light emitted from
    the Hubble radius will reach us in a
    finite amount of time.
    
    It is a common misconception that light
    from the Hubble radius can never reach us.
    
    It is true that particles on the Hubble
    radius recede from us with the speed of
    light, but the Hubble radius gets larger
    over time (because the Hubble parameter H
    gets smaller over time), so light emitted
    towards us from a particle on the Hubble
    radius will be inside the Hubble radius
    some time later.
    
    Only light emitted from the cosmic event
    horizon or further will never reach us in
    a finite amount of time.

event horizon
    [cosmological horizon]

    Differs from the particle horizon in that
    the particle horizon represents the
    largest comoving distance from which light
    could have reached the observer by a
    specific time, while the event horizon is
    the largest comoving distance from which
    light emitted now can ever reach the
    observer in the future.

future horizon
    [cosmological horizon]

    In an accelerating universe, there are
    events which will be unobservable as t
    approaches ∞ as signals from future events
    become redshifted to arbitrarily long
    wavelengths in the exponentially expanding
    de Sitter space.
    
    This sets a limit on the farthest distance
    that we can possibly see as measured in
    units of proper distance today.
    
    Or, more precisely, there are events that
    are spatially separated for a certain
    frame of reference happening
    simultaneously with the event occurring
    right now for which no signal will ever
    reach us, even though we can observe
    events that occurred at the same location
    in space that happened in the distant
    past.
    
    While we will continue to receive signals
    from this location in space, even if we
    wait an infinite amount of time, a signal
    that left from that location today will
    never reach us.
    
    Additionally, the signals coming from that
    location will have less and less energy
    and be less and less frequent until the
    location, for all practical purposes,
    becomes unobservable.
    
    In a universe that is dominated by dark
    energy which is undergoing an exponential
    expansion of the scale factor, all objects
    that are gravitationally unbound with
    respect to the Milky Way will become
    unobservable, in a futuristic version of
    Kapteyn's universe.

syzygy
    A co-alignment / lining-up of 3 bodies in
    gravitational alignment.

    Example:
    - An eclipse is a syzygy of the Earth, the
      Sun and the Moon.
    - When Jupiter is directly opposite the
      Sun from Earth.

          Jupiter - Earth - Sun.

      That makes it great for viewing.

apsis
    The farthest or nearest point in the orbit
    of a planetary body about its primary
    body.
    
    The apsides of Earth's orbit of the Sun
    are two:
    - the aphelion,
    - the perihelion.

aphelion
    Where Earth is farthest from the sun

perihelion
    Where Earth is nearest to the sun

sun dog
sundog
ock sun
parhelion
parhelia
    [#meteorology]

    An atmospheric optical phenomenon that
    consists of a bright spot to one or both
    sides of the Sun.
    
    Two sun dogs often flank the Sun within a
    22° halo.
    
    The sun dog is a member of the family of
    halos caused by the refraction of sunlight
    by ice crystals in the atmosphere.
    
    Sun dogs typically appear as a pair of
    subtly colored patches of light, around
    22° to the left and right of the Sun, and
    at the same altitude above the horizon as
    the Sun.
    
    They can be seen anywhere in the world
    during any season, but are not always
    obvious or bright.
    
    Sun dogs are best seen and most
    conspicuous when the Sun is near the
    horizon.