Edit in JSFiddle

var offsetTop = 10;
var dPerihelie = 147
var dAphelie = 152
var petitAxe = 149*2
var grandAxe = dPerihelie + dAphelie

var soleil = {x:dPerihelie, y:petitAxe/2 + offsetTop, rayon:10};
var terre = {rayon:5};
var centre = {x:grandAxe/2, y:soleil.y};

/*
var perihelie = {x:0,y:soleil.y};
var aphelie = {x:grandAxe, y:soleil.y};
*/

function ellipse(context, cx, cy, rx, ry){
    context.save(); // save state
    context.beginPath();
    
    context.translate(cx-rx, cy-ry);
    context.scale(rx, ry);
    context.arc(1, 1, 1, 0, 2 * Math.PI, false);
    
    context.restore();
    context.stroke();
}

function positionAuJour(context, cx, cy, rx, ry, jour){
    var rotationAngle =0;
    var i = (365 - jour)/365 * Math.PI * 2;
    
    xPos = cx - (rx * Math.sin(i)) * Math.sin(rotationAngle * Math.PI) + (ry * Math.cos(i)) * Math.cos(rotationAngle * Math.PI);
    yPos = cy + (ry * Math.cos(i)) * Math.sin(rotationAngle * Math.PI) + (rx * Math.sin(i)) * Math.cos(rotationAngle * Math.PI);
    
    return {x:xPos, y:yPos};
}

function dessineTerre(context, xPos, yPos){
    context.beginPath();
    context.arc(xPos, yPos, terre.rayon, 0, 2 * Math.PI, false);
    context.fillStyle = 'green';
    context.fill();
    context.closePath();
}

var jour = 0;

var draw = function(){
    var canvas = document.getElementById('solar-system');
    var context = canvas.getContext('2d');
    context.clearRect(0,0, 500, 320);
    document.getElementById("jour").value = jour;
    
    context.beginPath();
    context.arc(soleil.x, soleil.y, soleil.rayon, 0, 2 * Math.PI, false);
    context.fillStyle = 'yellow';
    context.fill();
    context.closePath();
    context.strokeStyle = 'blue';
    context.beginPath();
    ellipse(context, centre.x, centre.y, grandAxe/2, petitAxe/2);
    context.stroke();
    context.closePath();
    var position = positionAuJour(context, centre.x, centre.y, grandAxe/2, petitAxe/2, jour);
    dessineTerre(context, position.x, position.y);
    jour++;
    
    window.setTimeout(draw, 50);
}

draw();

<canvas width="500" height="320" id="solar-system"></canvas>

<input id="jour" type="text" value="0" />

#solar-system {
    margin-top:10px;
}