{
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   "source": [
    "Copyright (c) 2024 Massachusetts Institute of Technology\n",
    "\n",
    "SPDX-License-Identifier: MIT"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Constants and Celestial Positions\n",
    "\n",
    "`astroforge` makes some commonly used physical constants easy to access, and it also provides a simple lookup for the sun and moon positions given a time in Modified Julian Day (MJD) format."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Constants"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "The radius of Earth is 6378.137 km.\n",
      "The gravitational parameter of Earth is 398600.4418 km**3 / s**2.\n",
      "The oblateness of the Earth (J2 perturbation) is 0.0010826266751992014\n",
      "The rotation rate of the Earth is 7.292115147019296e-05 radians per second.\n",
      "\n",
      "The orbital radius of the geostationary belt is 42164.17236443124 km.\n",
      "The orbital velocity of a geostationary satellite is 3.0746599996020167 km/s.\n",
      "The primary acceleration for a geostationary satellite is 0.0002242077475503223 km/s**2.\n",
      "\n",
      "The speed of light is 299792.458 km/s.\n"
     ]
    }
   ],
   "source": [
    "from astroforge.constants import *\n",
    "\n",
    "print(f\"The radius of Earth is {R_earth} km.\")\n",
    "print(f\"The gravitational parameter of Earth is {GM} km**3 / s**2.\")\n",
    "print(f\"The oblateness of the Earth (J2 perturbation) is {J2}\")\n",
    "print(f\"The rotation rate of the Earth is {Omega} radians per second.\")\n",
    "\n",
    "print(\"\")\n",
    "\n",
    "print(f\"The orbital radius of the geostationary belt is {Rgeo} km.\")\n",
    "print(f\"The orbital velocity of a geostationary satellite is {Vgeo} km/s.\")\n",
    "print(f\"The primary acceleration for a geostationary satellite is {Ageo} km/s**2.\")\n",
    "\n",
    "print(\"\")\n",
    "print(f\"The speed of light is {c} km/s.\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "You can also find the gravitational parameters (in km<sup>3</sup>/s<sup>2</sup>) for other bodies in the solar system, including the combined Earth/moon system (`\"emb\"`):"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "{'mercury': 22032,\n",
       " 'venus': 324859,\n",
       " 'emb': 403503.23550216266,\n",
       " 'mars': 42828.375214,\n",
       " 'jupiter': 126712764.8,\n",
       " 'saturn': 37940585.2,\n",
       " 'uranus': 5793939,\n",
       " 'neptune': 6836529,\n",
       " 'moon': 4902.7779,\n",
       " 'sun': 132712440018.0}"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "ss_GM"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Sun and Moon Positions\n",
    "\n",
    "Given a time in MJD format, `astroforge` can calculate low-fidelity positions for the sun and moon in both [GCRS](https://docs.astropy.org/en/stable/api/astropy.coordinates.GCRS.html) and Mean Equator Mean Equinox of Date (MEMED) coordinates (see *Satellite Orbits* by Montenbruck & Gill, pg. 70)."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Approximate Moon Coordinates at MJD 60197.5:\n",
      "   GCRS:  [-184896.36188553  313416.42230858  175505.02631163]\n",
      "   MEMED: [-186957.66784405  312430.99946501  175077.79322196]\n",
      "\n",
      "Approximate Sun Coordinates at MJD 60197.5:\n",
      "   GCRS:  [-1.46913445e+08  3.06032318e+07  1.32681248e+07]\n",
      "   MEMED: [-1.47103679e+08  2.98240788e+07  1.29303206e+07]\n"
     ]
    }
   ],
   "source": [
    "MJD = 60197.5\n",
    "\n",
    "from astroforge.common import R_moon, R_moon_MEMED, R_sun, R_sun_MEMED\n",
    "\n",
    "r_moon_gcrs = R_moon(MJD)\n",
    "r_moon_memed = R_moon_MEMED(MJD)\n",
    "\n",
    "r_sun_gcrs = R_sun(MJD)\n",
    "r_sun_memed = R_sun_MEMED(MJD)\n",
    "\n",
    "print(f\"Approximate Moon Coordinates at MJD {MJD}:\")\n",
    "print(f\"   GCRS:  {r_moon_gcrs}\")\n",
    "print(f\"   MEMED: {r_moon_memed}\")\n",
    "\n",
    "print(\"\")\n",
    "\n",
    "print(f\"Approximate Sun Coordinates at MJD {MJD}:\")\n",
    "print(f\"   GCRS:  {r_sun_gcrs}\")\n",
    "print(f\"   MEMED: {r_sun_memed}\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "These utilities are especially useful for determining whether a satellite is too close to the sun or moon to be observed with an optical sensor."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": []
  }
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